Error-proof adaptive repair has been demonstrated in Pseudomonas aeruginosa. Cells of actively replicating wild-type Ps. aeruginosa (ATCC27853) and its auxotrophic derivative PAO 286 were subjected to stepwise adaptation (up to 1 μg ml-1) by nitrosoguanidine (MNNG). Such cells resisted lethal and mutagenic effects of MNNG-challenge (lethal) doses more efficiently than those of nonadapted cultures. Similarly, reactivation of alkylated Pseudomonas phages was enhanced in adapted cells only. Induction of adaptive repair enzymes was sensitive to chloramphenicol (protein synthesis-inhibiting antibiotic) during adaption treatment only. Protein extract from adapted cells showed increased levels in sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE).