The toxic hazards of exposure to alpha-naphthylamine (134327) (1-NA) are discussed. Its use in the manufacture of azo dyes and in the rubber industry is noted, and the phenomenon of aniline cancer is described. The concentration of beta-naphthylamine (91598) (2-NA) found in industrial 1-NA is discussed. The results of 1-NA carcinogenicity tests in rabbits, rats, dogs, mice, and hamsters are summarized, along with species differences in 1-NA metabolism. The relative carcinogenicity of N-hydroxylated derivatives of naphthylamines is reviewed, including tumor induction rates for 1- naphthylhydroxylamine (607307) and 2-naphthylhydroxylamine (613478). The carcinogenic potentials of the 1 nitroso and 2 nitroso compounds of 1-NA and 2-NA are compared. Findings from several epidemiological surveys of workers exposed to 1-NA are discussed, including occurrences of sessile papillomas and bladder tumors in relation to duration and degree of exposure. The possible interaction between 1-NA and 2-NA in inducing bladder tumors is considered. The mutagenicity of the N-hydroxylated derivatives is described along with the relationship between carcinogenicity and mutagenicity. The author notes that the contamination of 1-NA by 2- NA precludes a direct evaluation of the carcinogenicity of 1-NA alone. Both compounds yield metabolites that have been determined to be proximate carcinogens; however, the ultimate carcinogenic metabolite remains unknown. 1-NA should be considered a human carcinogen until evidence to the contrary is established.