The hazards of 4-aminodiphenyl (92671) (ADP) are reviewed. Experiments are reported in which dogs given various doses of ADP develop urinary bladder tumors. Comparison of relative potency of tumor induction to other bladder carcinogens shows that ADP is at least as active a bladder carcinogen in dogs as beta-naphthylamine (91598) and considerably more potent than benzidine (92875) or 2- acetylaminofluorene (53963). The importance of avoiding skin contact and inhalation of vapors and dusts is stressed. In rats, ADP induced tumors in liver, intestine, and mammary glands. Four dogs given oral capsules containing ADP were diagnosed as having bladder carcinomata. It is reported that in other experiments on dogs, rabbits, and mice ADP results in production of bladder carcinomas or other types of tumors. The importance of N- hydroxylated metabolites of ADP is noted. Bladder cancer was reported in 11.1 percent of 171 workers engaged in the production of ADP for 1 to 19 years. The author concludes that the carcinogenicity of ADP in rabbits, mice, and dogs is well established from the literature. Of greater significance is its clear implication in the induction of bladder tumors in workers engaged in its production.