ASSAY OF IMIDAZOLIDINYL UREA 317 Table I Antimicrobial Activity Supplier Sample A B C D E 1 75% 110% 81% 95% 95% 2 60% 100% 83.8% 100% 95% 3 75% 95% 83.8% 105% 100% 4 70% 105% 75% 95% 90% 5 100% 100% 75% 100% 95% 6 55% 95% 90% 90% 100% 7 90% 105% 80% 95% 90% 8 65% 95% 80% 100% 95% Table II Product • o' o'/X•n A 73.75 15.5 I 5.33 B 100.62 5.62 I 1.99 C 81.07 4.93 I 1.74 D 97.7 4.63 I 1.64 E 95 3.78 I 1.34 Average. Standard deviation. tr/•nn = Standard error. Dilution factor. two drums of A from the same production lot are extremely high (o- = 15.5), which suggests a problem in using this source of product as a preservative. CONCLUSIONS The activity of the preservative imidazolidinyl urea was determined by a method based upon the capacity of the substance to inhibit the growth of P. aeruginosa ATCC15442 in an agar diffusion assay. Under standard conditions (inoculum, medium, incubation time) the diameters of the inhibition zones are proportional to the logarithms of the preservative concentration. The method is very simple and doesn't require expensive equipment. The accuracy and reproducibility are acceptable. This routine analysis is applicable to and useful for exactly adjusting the concentration of active preservative in a formulation to obtain the antimicrobial activity desired. This method could be applied to determine the activity of other preservatives such as kathon ©, glydant ©, etc. on the condition that they have sufficient diffusibility in water. Studies concerning this possibility are being carried on.

318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS REFERENCES (1) D. Baylocq, J. C. Billiotte, P. Bourrinet, M. Hamon, F. Pellerin, l•tude de deux conservateurs utilis6s en cosm6tologie: Bronopol et Germall 115, Labo-Pharma Problems et Techniques, 286, 332- 335 (April 1979). (2) Sutton Laboratories, GermM/115 s]9ecification methods, (personal communication), Roselie, New Jersey. (3) D. F. Spooner and G. Sykes, "Laboratory Assessment of Antibacterial Activity," in A•ethods in Rlicrobiology, J. R. Norris and D. W. Ribbons, Eds. (Academic Press, London and New York, 1972), Vol. 7B, pp 215-233. (4) L. Cavalli-Sforza, "Dosaggi a Rette Parallele," in AnMisi St•tistic• ]9er 34edki e Bio/ogi, (Boringhieri, 1980), pp 113-137.