RoUS AND SMITH (1945) drew attention to the fact that embryo skin of BALB/c strain mice if minced and inoculated into the thigh muscle of adults of the same strain would survive and differentiate, and that by including methylcholanthrene in the inoculum, squamous carcinomas could be induced in the implantation cysts. The same procedure failed to yield such clear-cut results using other strains of mice. Our procedure for short term testing of substances for carcinogenic potential derived from these observations, is described below. It is essentially simple and we feel it is worth mentioning certain manipulations which permit a rapid turnover of material. Our experiments also confirm that " c " mice are the most satisfactory to use. MATERIAL AND METHODS Source of implant material BALB/c strain female mice were segregated into individual boxes when obviously pregnant, and when within 48 hours of term were killed by cervica
