Binding of penicillin to the DD-carboxypeptidase of the unstable spheroplast L-form of Proteus mirabilis results in the rapid formation of a modified enzyme-inhibitor complex which in turn undergoes rapid decay into reactivated enzyme and an antibiotically inactive penicillin degradation product. Major antibiotic metabolites recovered from such interactions were benzylpenicilloic acid and phenoxymethylpenicilloic acid from benzylpenicillin and phenoxymethylpenicillin, respectively, suggesting a second enzymic function of the DD-carboxypeptidase as a penicillinase of low efficiency. Statistical analyses made with the help of a linear regression program show that the enzyme interacts with the substrate UDP-N-acetylmuramoyl-L-alanyl-D-gamma-glutamyl-(L)-meso-2,6-diaminopimelyl-(L)-D-alanyl-D-alanine and either benzympenicillin or carbenicillin in a non-competitive manner.