The mutagenicities of dimethylnitrosamine (DMN) and diethylnitrosamine (DEN) on Chinese hamster V79 cells were examined by cocultivation of the cells with primary cultures of rat hepatocytes or hamster hepatocytes. Mutations in V79 cells were selected by resistance to ouabain. The mutation frequencies induced by all doses of DMN (1, 5, 10 and 50 micrograms/ml) and DEN (10, 50, 100 and 500 g/ml) were substantially higher on cocultivation with hamster hepatocytes than with rat hepatocytes. Thus it is proposed that hamster hepatocytes are useful for detecting mutagenesis in V79 cells induced by nitrosamines.