Rats which over a 1-year period ingested diets containing a technical grade mixture of dinitrotoluenes (TDNT), primarily composed of the 2,4- and the 2,6- isomers, or which were fed only 2,6-dinitrotoluene (2,6-DNT) developed cancers of the liver. The liver cancers consisted of hepatocellular carcinomas and in some cases cholangiocarcinomas. Two-year TDNT ingestion studies in rats produced subcutaneous fibromas and fibrosarcomas, mammary fibroadenomas, and the liver cancers. Male mice fed TDNT for 2 years developed papillary and cortical carcinomas of the kidney and nonmalignant kidney tumors diagnosed as papillary and cortical adenomas. Rats fed 2,4-dinitrotoluene (2,4-DNT) throughout a 2-year carcinogenicity study developed statistically significant incidences of subcutaneous fibromas and mammary fibroadenomas and low incidences of hepatocellular carcinomas and subcutaneous fibrosarcomas which were not statistically significant. Feeding or oral administration of TDNT or 2.6-DNT for up to 2 years induced decreased spermatogenesis, aspermatogenesis, or testicular atrophy in dogs, rats, or mice. Nonfunctioning ovaries were found in mice fed TDNT for 2 years. Based on these data, the National Institute for occupational Safety and Health (NIOSH) recommends that TDNT and 2,6-DNT be regarded as potential human carcinogens in the workplace and possible inducers of adverse reproductive effects. Although there is limited evidence indicating that 2,4-DNT poses a risk to human health, the existing animal and in vitro data are suggestive of such a potential. The potential of the other DNT isomers to induce adverse health effects has not been determined, but the probability of developing such effects would be decreased by reducing exposure. Therefore, NIOSH recommends that occupational exposures to TDNT and the isomers of DNT be controlled to the fullest feasible extent.