The proteins that specifically bind penicillin G were studied with Caulobacter crescentus. This organism possessed at least 5 penicillin-binding proteins (PBPs): PBP lA (132 K), PBP 1Bs (98 K), PBP 2 (77 K), PBP 3 (64 K), and PBP 4 (50 K). As expected from the unique mor-phology, the C. crescentus PBPs were different from those of other sources in molecular weight and localization: C. crescentus did not possess PBPs of low molecular weight. PBP 4 was found only in the outer membrane, while the other PBPs were present mostly in the inner membrane.