Carcinogenicity of the fungicide fenaminosulf (p‐dimethylami‐nobenzenediazosodium sulfonate) and one of its bacterial metabolite, dimethyl‐p‐phenylenediamine (DMPDA) was evaluated in rats. Over a 17 month period, rats fed with 250 mg of the compounds per kg of feed (in the presence or absence of nitrite) did not develop tumors. Phenols were detected in the urine and faeces of rats receiving either fenaminosulf or DMPDA suggesting possible detoxification of these compounds. These compounds being positive in mutagenicity screening tests in the bacteria, in the light of our present studies it is suggested that testing potential carcinogens in experimental animals is a pre‐requisite before arriving at definitive conclusions on the carcinogenicity of chemicals.