The two structural isomers 2,4- and 2,6-diarninotoluene (DAT) differ in their carcinogenic properties; the 2,4-isomer is carcinogenic in rats and mice, whereas the 2,6-isomer has been reported to be non-carcinogenic. Both isomers were reported to be mutagenic inSalmonella typhimuriumin the presence of S9, which was confirmed in the present study beforein vivoassays were commenced. Both isomers were tested in the rat bone marrow micronucleus test and the rat liver UDS test to investigate how well these assays discriminate between the carcinogenic and the non-carcinogenic isomer. In the micronucleus test both isomers gave weakly positive results; however, with the carcinogen 2,4-DAT this weak effect was only detectable at very toxic doses and therefore the biological relevance of this result is questionable. Thus, the micronucleus test did not discriminate correctly between the carcinogen and the non-carcinogen. With the liver UDS test, discrimination was achieved but the positive effect seen for the carcinogenic isomer was weak and dependent on the method of preparation of the dosing suspensions. The results are discussed in relation to the carcinogenicity data on both compounds. It is concluded that although both isomers are potent genotoxinsin vitrothey exert their genotoxic potential only weaklyin vivoand convincing discrimination between the carcinogenic and non-carcinogenic isomer was not demonstrated.