The mutagenicity of 15 household dog urine specimens were measured by the combination of blue rayon extraction and ultramicro forward-mutation method with Salmonella Typhimurium TM677 strain. A good dose-response relation was observed between the urine volume and mutation frequency. The minimum amount of urine required was 20 ml or less. The specific mutation frequency of urine greatly varied from one dog to another. The average specific mutation frequencies in the presence and absence of S9 mix were 28.7 +/- 51.5 (x 10(-4)) and 12.0 +/- 13.3 (x 10(-4)), respectively, and there was no significant difference between them. The mutation frequency markedly increased after the ingestion of broiled fish. Ten human urines specimens showed a similar level of specific mutation frequency to that of the dog urine specimens in both the presence and absence of S9 mix.