The mechanisms underlying cellular resistance to the antitumor drug cis-diamminedichloro-platinum(II) (CDDP) were studied in Escherichia coli K12. A bacterial strain (MC4100/DDP) was selected from the MC4100 wild-type strain after growth for four cycles in CDDP. MC4100/DDP bacteria showed a high level of resistance and exhibited various modifications including (1) a decrease in drug uptake and platinum/DNA binding which only partly contributed to resistance, (2) an increase in glutathione content not involved in the resistant phenotype, (3) an increase in DNA repair capacity. Resistance was unmodified by introducing a uvrA mutation which neutralizes the excision-repair pathway. In contrast, it was abolished by deletion of the recA gene which abolishes recombination and SOS repair but also by a mutation in the recA gene leading to RecA co-protease minus (no SOS induction). RecA protein was unchanged in MC4100/DDP but the expression of RecA-dependent gene(s) was required for CDDP resistance. The regulation of genes belonging to the SOS regulon was analysed in MC4100/DDP by monitoring the expression of sfiA and recA::lacZ gene fusions after UV irradiation. These gene fusions were derepressed faster and the optimal expression was obtained for a lower number of UV lesions in MC4100/DDP, suggesting a role of RecA co-protease activity in the mechanism of resistance to CDDP in this E. coli strain.