The hazards of occupational exposure to beta-naphthylamine (91598) are reviewed. The epidemiological pitfalls of determining carcinogenicity of the dye are outlined. These are: worker exposure to more than one suspect compound, complicated by shifting of workers between departments; different degrees of exposure hazard between processes; unsuspected impurities in trace amounts that are possibly more harmful than the parent compound; and differences in composition of dyes and production methods in different factories that complicate statistical comparisons. Animal experiments are cited in which dogs given subcutaneous injections or fed the dye developed bladder tumors. Bladder papillomas, hepatomas, bronchogenic carcinomas, and lung carcinomas produced in Wistar-rats are noted. Proof of carcinogenicity of the dye in mice, monkeys, and hamsters is also presented. Fifteen metabolites identified for beta-naphthylamine are listed. An epidemiological study is described in which 109 of 376 workers exposed to beta-naphthylamine and other aromatic amines in a dye factory had bladder malignancies. Of 54 of these workers exposed to beta-naphthylamine, 31.5 percent had bladder malignancies. The latency period ranged from 6 to 38 years; mixed exposures to beta-naphthylamine and benzidine resulted in 50 percent bladder malignancies. The author concludes that the carcinogenicity of beta-naphthylamine is well established by both animal data and human experience. It is clearly implicated as a highly hazardous substance in occupational environments.