The genotoxic effects of the methylating agent streptozotocin (STZ) on Chinese hamster cells CHO-9 and V79 were evaluated. The induction of cell killing, chromosomal aberrations, sister-chromatid exchanges (SCEs) and mutations was analyzed. Comparisons were made with the the STZ aglyconic analogue N-methyl-N-nitrosourea (MNU). V79 cells were found to be more resistant than CHO-9 cells to STZ and MNU killing effects, as well as to the induction of chromosomal aberrations and SCEs; however, V79 and CHO-9 cells appeared to be equally sensitive to the induction of 6-thioguanine resistant mutants by STZ. These results suggest that an error-free mechanism that tolerates DNA methylation damage confers a resistant phenotype to V79 cells to the genotoxic effects of methylation damage.34727985