Whenever possible, at least one hundred mice of each sex of each of the fifteen strains were used (Table 1). The animals were accumulated slowly and the experiments extended over a period of two years (1942-45). One-tenth cc. of sesame oil containing 1 mg. of methylcholanthrene was injected into the right flank of all mice at the age of 60 ± 4 days. As a rule 5 to 8 mice were kept in a box and ...