Studies were made on the urinary bladder of 66 BALB/c mice of both sexes; 21 were fed a normal diet and 45, a diet containing 100-500 parts per million (ppm) of 2-acetylaminofluorene (2-AAF). Bladder epithelial hyperplasia was noted in 22 of 24 mice given 2-AAF for 7 months and in 20 of 21 given 2-AAF for 19 months. Transitional cell carcinomas were in 12 of 21 mice given 2-AAF for 19 months and in all except the 9 females given the low dose (100 ppm). Special stains revealed marked alkaline phosphatase activity in the bladder epithelium of untreated controls and slightly to moderately reduced activity in the hyperplastic epithelium, particularly the lower layers, of mice given 2-AAF for 7 months. Activity was markedly reduced and confined largely to the upper layers in mice treated for 19 months. The transitional cell carcinomas showed no activity except focally in a few tumor masses in 11 of 12 mice with tumors. Alkaline phosphatase activity was usually slightly to moderately increased in the sub-epithelial stroma after 7 months' treatment with 2-AAF and markedly increased after 19 months. Loss of alkaline phosphatase activity in the bladder epithelium may be a preneoplastic change.