The toxic hazards of benzidine (92875) are reviewed. The physical forms of benzidine are described along with its use in azo dyestuffs and its method of manufacture. Metabolic products found in exposed rats, dogs, mice, rabbits, and humans are identified, with 3- hydroxybenzidine noted as the primary metabolite in all species. The increased incidence of bladder cancer among workers in the aniline dye industry is discussed in relation to exposure to benzidine and an intermediate product, beta-naphthylamine (91598). The findings of numerous carcinogenicity studies are summarized, including subcutaneous, oral, dermal, and inhalation exposure tests in rats, mice, rabbits, dogs, and hamsters using technical grade and high purity benzidine, benzidine-sulfate and benzidine-dihydrochloride (531851). Epidemiological data also is summarized from human exposure cases. Bladder cancer incidence data is included for workers who manufactured aniline dyestuffs, workers in a coal tar dye factory, and workers who manufactured benzidine and beta- naphthylamine. Also provided is data on average latency period, survival rates, and incidence of malignant tumors in organs other than the bladder. The author concludes that benzidine and its salts have been proven to be human carcinogens. Maximal protection should be provided to all workers who are exposed to this chemical.