The aromatic amine 2,4-toluenediamine was fed at levels of 50 and 100 ppm to inbred, barrier-raised F344 rats for 2 years. The high dose induced a statistically significant increase in the incidence of hepatic neoplasia in males, and it induced a significant dose-related positive trend in the incidence of liver neoplasms in both sexes. Hepatocellular changes considered to be associated with neoplasia were increased at a high level of statistical significance in both sexes. In addition, the compound caused statistically significant increases in the incidence of mammary tumors in females and an increase of mammary tumors in males which, although not significant statistically, was nevertheless considered to be related to the chemical. The compound was hepatotoxic and accelerated the development of chronic renal disease in this strain, an effect that contributed to a marked decrease in the survival of dosed animals.