The comparison of a clinical Acinetobacter baumanii isolate (strain No. 4852/88) and its selected imipenem-resistant (IMR) clone exhibited a complex reorganization of the penicillin-binding proteins (PBPs) with diminished labelling of all PBPs except the 24-kD PBP which showed an increased binding of 14C-penicillin. This protein could not be saturated by preincubation of membranes with imipenem at 8-fold the MIC of imipenem, thus indicating PBP alterations responsible for imipenem resistance. In A. baumanii 4852/88 seven PBPs with the apparent molecular weights of 94, 84, 65, 61, 48, 40 and 24 kD could be detected. beta-Lactamase production was barely detectable in any case and could not be enhanced in the presence of various beta-lactams as the inducer. The outer membrane proteins were found identical in both the wild-type strain and the Im clone. So far, imipenem-resistant A. baumanii isolates have been isolated twice in our diagnostic laboratory; however, no implications on the future relevance of the above findings can be made.