All N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistant (MR) cell lines of HeLa S3 Mer- cells tested were found to be defective in O6-methylguanine-DNA methyltransferase and more sensitive than HeLa Mer+ cells to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-1-nitroso-ure a, like the parental HeLa S3 Mer- cells. MR10-1 and MR1-3 cells were also highly resistant to induction of sister chromatid exchanges (SCEs) by MNNG, while MR6, MR8 and MR11 cells were slightly resistant to SCE induction, MR10-1 and MR1-3 cells showed higher spontaneous mutation rates than the parental cells. These results indicated that there are at least two types of MR cells. All MR cells were more sensitive than HeLa S3 Mer- cells to 6-hydroxyamino purine.