Groups of 52 B6C3F1 mice of each sex were maintained on a diet containing 1.25 or 0.6% phenacetin for 96 weeks and then fed a basal diet for 8 weeks. Control groups consisted of 50 mice of each sex and were fed a basal diet for 104 weeks. All animals were killed at the end of the experiment and all organs were examined histopathologically. Mice that died during the experiment were also autopsied and those that survived for more than 57 weeks, when the first tumor was observed, were also included in the effective number of mice. Tumors were found in the kidney, liver, lung, skin, hematopoietic system (leukemia or lymphoma) and occasionally in some other organs. The dose-related induction of renal cell tumors in the male mice fed phenacetin was clearly demonstrated in this experiment. Urinary bladder lesions that developed in the mice of either sex fed 1.25% phenacetin were also considered to be due to the tumorigenicity of phenacetin. Tumors of other organs in either the phenacetin-treated or the control group were regarded as strain-related spontaneous tumors of B6C3F1 mice.