The current state of mutagenicity testing 211 (d) be reliable and reproducible, (e) be economically viable and quick. No one test system at present meets all these demands. Below the test systems that are available are considered (References are given for the test systems only where systems are not generally discussed later.) Gene mutations may currently be detected by: (a) micro-organisms with metabolic activation in vitro and in vivo, (b) Drosophila melanogaster, (c) cultures of somatic mammalian and human cells, (d) specific locus tests in mammals (48, 49). Cytogenetic effects in somatic cells can be detected by: (a) the micronucleus test, (b) conventional metaphase analysis or sister chromatid exchange analysis (50) of bone marrow cells from mammals and mammalian and human lymphocyte cultures. For detecting chromosomal damage in spermatogenesis and oi3genesis, the dominant lethal or heritable translocation test (30, 51, 52) can be used. Drosophila can also be used for detecting chromosomal damage, and since Drosophila can also detect gene mutation it is a useful 'catch-all' system (53, 54) but extrapolition to man is difficult from such an organism. In addition to methods that detect gene mutations and chromosome aberrations, the potential of a chemical to induce primary DNA damage can be detected by measuring stimulation or inhibition of repair (55, 56, 57), or recombinational or gene conversion events (58, 59). It is not feasible for all chemicals to be tested by all methods, so a system of priorities should be arranged. Not all substances are used by man in the same way: there are some substances to which man is chronically exposed and others to which man is only subjected by acute exposure under exceptional circumstances. Thus the genetic risk of a substance will depend on its mutagenic potency, where it is known, the extent to which people come into contact with it, and on the individual susceptibility of a person. We have to accept the fact there is variation in individual susceptibility but we can differentiate our test systems according to levels of exposure of the population to a chemical. Exposure will depend on a combination of two parameters: the number of people exposed and the dosage to which the people are exposed. If the product of the parameters is low then the chemical is a low exposure chemical: if the product is high then the chemical is a high exposure chemical. Low exposure chemicals, such as very low tonnage industrial chemicals including some intermediates, non-ingested substances and substances known not to accumulate in the environment or body, may be subjected to a simpler screening programme, which should, however, cover the induction of both gene mutations and chromosome aber- rations, e.g. a test on micro-organisms with metabolic activation in vitro and a cytogenetic examination of bone marrow cells, or an in vitro cytogenetic examination with metabolic activation. High exposure chemicals, such as high tonnage industrial chemicals, pesticides, widely-used medicine, food additives, ingested products and substances known to accumulate, should be subjected to more rigorous testing with at least the two tests

212 Diana Anderson mentioned above and a relevant whole mammal test in germ cells. Other tests may be added if the chemical warrants it, such as cytogenetic analysis of human lymphocytes of exposed workers, or any others that may be considered relevant. Substances found positive in the first screening, depending on their economic or medical importance, could be subjected to more rigorous testing. According to Matter (19), however, very few compounds (0'1•o) originally produced in pharmaceutical re- search ever have a chance of being marketed. He suggests that extensive early muta- genicity testing of drugs is of little help since benefit/risk decisions cannot be made at that stage, and that such testing should be concentrated on those compounds scheduled for clinical trials or commercial introduction. However, it would seem better to avoid mutagenic products at an early stage where possible. Before a substance is tested, consideration should be given to its chemical structure, to determine if it is related to compounds which already have known mutagenic, carcino- genic, teratogenic or general toxic effects. Such a consideration might give an indication of its mutagenic potential and this may be useful in setting testing priorities. If it is suspected of having high mutagenic potency, then this factor may override the other two parameters which determine exposure. COMMENTS ON SOME OF THE METHODS A great deal has already been published about recommended protocols, e.g. (17, 60). However, some general rules apply for all testing systems. All assays should be run with concurrent negative and positive controls. Positive controls should, where possible, be structurally or mechanistically related to the compound under test. MICROBIAL METHODS Much work in different laboratories has been carried out on the bacterial Salmonella typhimurium mutagenicity system with rat liver microsomal activation, and as such it is well validated both for the detection of some mammalian and human carcinogens and bacterial mutagens. However, it is not well validated for human germ cell mutagens since none are unequivocally known. This does not mean that other microbial systems would not be equally valid for detecting mutagens/carcinogens if as much work had been undertaken with them. The Salmonella typhimurium strains of Ames (3) are able to detect base-pair substitution and frameshift gene mutations. A drawback to the system is that it is not quantatitive and the potency of mutagens cannot be ranked. However, the system is useful for a qualitative answer. The system of Salmonella typhimurium and other systems such as that of the bacterium Escherichia coli (61) and yeasts (62, 58, 59) can provide a quantitative answer if used in liquid cultures in combination with viability studies. Yeasts, however, are used to measure not only gene mutation but more commonly gene conversion and mitotic recombinational events. A very sensitive test for measuring gene mutation is claimed when E. coli is used in a fluctuation assay (63). Generally, compounds in a screening programme should be tested both with and without microsomal activation. Strains to detect both base-pair and frameshift mutations should be used, as well as at least five concentrations of the test compound over a wide dose range in order to maximise the chance of obtaining a response, with the highest dose if possible inhibiting the growth of the microbes. This ensures that the compound has entered the microbes. Replicate experiments should be undertaken to determine a