210 Diana Anderson may, therefore, disturb normal bodily functions producing a change in one of these parameters to cause a genetic effect indirectly. TEST METHODS AND RECOMMENDATIONS FOR THEIR USE Geneticists have a large number of experimental methods at their disposal to assess the mutagenic action of chemicals. No single method, however, gives any conclusive in- formation about the genetic risk to a person who has been exposed to a mutagenic substance. Thus it would seem necessary for a number of test systems to be used. Bochkov et al. (17) have listed the effects, and given a useful pr6cis of the results of different types of chemicals such as cytostatics, antibiotics, pharmaceutical preparations, food additives and admixtures, chemosterilants. fungicities, insecticides, herbicides and industrial chemicals in the different test systems of micro-organisms, plants, fruit fly (Drosophila), mammalian cell cultures, bone marrow and dominant lethal systems of mammals and human cells in cultures and human lymphocytes. Various recommendations for combinations of test systems have been put forward (35-46, 17). All approaches have their advantages and disadvantages. In the more recent reports Bridges (43) proposed that assessment at each stage of testing should take into account the population exposed to the substance, its economic value and the possibility of substituting a non-mutagenic grouping for the mutagenic grouping in the structure of a substance. However, non-mutagenic substitutions are not always possible without altering the desired biological activity of a product. The testing scheme proposed by Flamm (45) includes tests for the detection of heritable translocations and specific locus mutations. Both of these tests require large numbers of animals, are not always economic- ally feasible, and in any case do not necessarily generate sufficient information to assess the genetic risk to man. Various governmental agencies, such as those of America, Japan, Britain and other EEC countries, are now preparing guidelines for testing methods. Some of the more recent ideas indicate the need for flexibility. This should permit changing to new and improved methods as soon as their usefulness has been substantiated. However, there is anxiety that any significant deviation from generally accepted guidelines may be questioned by regulatory agencies and that the guidelines that will satisfy the most demanding health authority may become the generally accepted procedure. With all the recommendations in the communications already in the literature (listed above), those put forward in this present communication will be brief, and it must be remembered that testing organisations, if allowed flexibility, will have their own ideas regarding scientific protocols, etc. The problem with the present testing methods is that not all systems are equally reliable and reproducible. Even the systems in which most compounds have been tested, such as the Salmonella typhimurium plate incorporation mutagenicity assay with meta- bolic activation (5), give different results for some compounds in different laboratories (47). Ideally, a system for assessing chemicals for mutagenic activity should: (a) detect the potential of the test substance to induce gene and chromosome mutations in somatic and germ cells, (b) provide quantitative data for extrapolation to man, (c) be capable of detecting metabolic products of the compound which have mutagenic potential,

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