A mutant of Staphylococcus aureus strain 655 was isolated that is restricted in penicillinase induction. Wild-type plasmids that bear penicillinase determinants could not be fully induced in this mutant, 655par-1; hence, the responsible mutation is not located on the plasmid. Mutant plasmid PI(258)penI443, which produces penicillinase constitutively in wild-type cells, was fully constitutive for penicillinase production when it was harbored by mutant 655par-1. Therefore, the bacterial mutation does not interfere directly with the transcription of the penZ gene or translation of the penicillinase messenger ribonucleic acid. Mutant plasmid PII(147)penI220 was fully inducible in the mutant bacterium, even though the wild-type plasmid PII(147) was only partially inducible in the par-1 mutant. Thus, in the presence of inducer, complementation appears to occur between the product of the par-1 gene and the product of the penI220 gene. These results suggest that the par-1 gene codes for a penicillinase antire-pressor.