A concise assessment of the risks to human health posed by exposure to ortho-toluidine, a chemical used in the manufacture of dyestuffs and, to a lesser extent, in the production of rubber, chemicals and pesticides, and as a curing agent for epoxy resin systems. The document is part of a new series of brief reports aimed at the characterization of hazards and dose-response for exposures to selected industrial chemicals. With this goal in mind, documents in the series focus on studies and findings considered critical for risk characterization. The document is primarily concerned with assessing the carcinogenic risk in occupationally exposed workers. A review of findings from studies in rats and mice, where oral administration of o-toluidine has been linked to a significant increase in the incidence of benign and malignant tumors, supports the conclusion that o-toluidine may act as a genotoxic carcinogen. In humans, limited epidemiological evidence suggests an increased risk of bladder cancer in occupationally exposed workers. In view of evidence that the chemical's carcinogenic action involves a genotoxic mechanism, the assessment was unable to identify a threshold at which exposure would not result in some risk to human health.