The role of nucleotide excision repair and 3-methyladenine DNA glycosylases in removing cytotoxic lesions induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in Salmonella typhimurium and Escherichia coli cells was examined. Compared to the E. coli wild-type strain, the S. typhimurium wild-type strain was more sensitive to the same dose of MNNG. Nucleotide excision repair in both bacterial species does not contribute significantly to the survival after MNNG treatment, indicating that the observed differences in survival between S. typhimurium and E. coli should be attributed to DNA-repair systems other than nucleotide excision repair. The survival of the E. coli alkA mutant strain is seriously affected by the lack of 3-methyladenine DNA glycosylase II, accentuating the importance of this DNA-repair enzyme in protecting E. coli cells against the lethal effects of methylating agents. Following indications from our experiments, the existence of an alkA gene analogue in S. typhimurium has been questioned. Dot-blot hybridisation, using the E. coli alkA gene as a probe, was performed, and such a nucleotide sequence was not detected on S. typhimurium genomic DNA. The existence of constitutive 3-methyladenine DNA glycosylase, analogous to the E. coli Tag gene product in S. typhimurium cells, suggested by the results is discussed.