To examine the relationship between altered spontaneous mutation rates and malignant characteristics of cells, two hydroxyurea-resistant Chinese hamster ovary cell lines, with alterations in ribonucleotide reductase, were examined for their rates of spontaneous mutation to 6-thioguanine and ouabain resistance, tumor growth rates and their ability to form experimental lung metastases. The most resistant cell line, HR-R2T, showed no changes in the rate of spontaneous mutation to 6-thioguanine or ouabain resistance compared to the parental wild-type cell line; however, the mutant line formed lung metastases in experimental metastasis assays with BALB/c nu/nu mice, and exhibited metastatic abilities significantly different from the wild-type population. Furthermore, the HR-R2T population did not show imbalances in any of the deoxyribonucleoside triphosphate pool sizes, which are frequently observed in cells altered in ribonucleotide reductase activity. The second hydroxyurea-resistant line, HNR-AT, had gross alterations in dCTP and dGTP pools and although the rate of spontaneous mutation to 6-thioguanione resistance was unaltered, it showed a moderate decrease in the rate of spontaneous mutation to ouabain resistance when compared to the parental wild-type population. Interestingly, the HNR-AT cell line did not form any lung metastases in the experimental metastasis assay. Both mutant cell lines, HR-R2T, and HNR-AT, had increased tumor growth rates in C57 BALB/c "beige" nude (nu/nu) mice as compared to the parental wild-type population. In total, the results obtained with the two mutant cell lines question the association of altered mutation rates with increased metastatic potential. Although several explanations are possible for the altered malignant properties exhibited by HR-R2T and HNR-AT cells, it is interesting to note that the results are consistent with earlier suggestions that changes in ribonucleotide reductase may accompany modifications in the malignant characteristics of cells.