Intact pZ189 DNA was replicated in monkey kidney vero cells which had been pretreated with Nmethyl-N’-nitro-N-nitrosoguanidine （MNNG） . The mutants were selected in E. coli MBM7070 and the mutation frequencies involving mutants with unchanged electrophoretic mobilrty of their plasmid DNA were scored. When compared to the spontaneous mutation frequency. the mutation frequencies were increased by 5.8 and 2.9-fold in cells pretreated with 0. 2 and 2μmol/L MNNG

respectively. The supF genes of these mutants were sequenced. and it was found that the types of base substitution and the sites of frameshifts differed from findings in studies of spontaneous and targeted mutagenesis. The results suggest that nontargeted mutagenesis occurs in mammalian cells and may have a sequence specificity.