To investigate the hypothesis that increased malignant potential correlates with increased genetic instability, we measured spontaneous mutation rates for the production of ouabain-resistant mutants in two benign (nonmetastatic) murine cell lines and their recently induced metastatic variants. Metastatic variants of the NIH 3T3 and CBA SP-1 cells were induced by transfection with the h-ras oncogene. Metastatic variants were also induced from the CBA SP-1 cell line by treatment with either 2'-deoxy-5-azacytidine or hydroxyurea. Mutation rates for the parent NIH 3T3 cells and their metastatic variants were less than 3 X 10(-8) variants per cell per generation, with no significant differences between them. Rates for the CBA SP-1 line and its variants ranged from 9 X 10(-9) to 8 X 10(-8) variants per cell generation, again without statistically significant differences. We conclude that in the cell lines studied the rate of spontaneous mutation for ouabain resistance was unrelated to the acquisition of the metastatic phenotype. This conclusion was based on the view that the generation of ouabain-resistant mutants is a reflection of the overall stability of the genome. Since the spontaneous mutation rate for ouabain resistance was unchanged in cells that had recently acquired the ability to metastasize, other genetic or epigenetic events were probably responsible for progression to the malignant (metastatic) phenotype.