The hazards of exposure to 4-nitrobiphenyl (2395995) (NBP) are reviewed. Incidences of bladder cancer in 11.1 percent of workers are cited as part of an epidemiological study of workers involved in the production of 4-aminodiphenyl (92671) (ADP), which was produced from 1935 to 1955 in the United States. The carcinogenicity of both compounds is stressed, and both are noted as probably responsible for the tumors reported in workers. A study is reported for administration of NBP 3 times weekly in four dogs, three of which developed malignant bladder tumors. The conclusion is cited that based on experiments in dogs, NBP is as potent a carcinogen as ADP. Other experiments in dogs are discussed in which lower doses of NBP or doses in combination with 2-napthylamine (91598) (NA) are given. The relative carcinogenic potency of ADP, NA, NBP, and benzidine (92875) are given as 1, 6, 17, and 27, respectively. Evidence is presented for in-vivo reduction of NBP to ADP and 4-amino-3-biphenyl- hydrogen-sulfate. The author concludes that the case for carcinogenicity of NBP is supported by the induction of bladder carcinoma in dogs, the evidence that NBP is metabolized to ADP, and the possibility that human cases of bladder cancer may have been induced by exposure to NBP as well as to ADP.