Penicillin-binding proteins of three Streptomyces strains, S. cacaoi, S. olivaceus, and S. clavuligerus, were examined by gel electrophoresis and fluorography. In a beta-lactamase producer, S. cacaoi, at least five membrane-bound penicillin-binding proteins were detected, but in two beta-lactam producers, S. olivaceus and S. clavuligerus, fewer penicillin-binding proteins were detected. Mecillinam and methicillin bound selectively to some penicillin-binding proteins in S. cacaoi, whereas they did not bind at all to those in S. olivaceus and S. clavuligerus. Clavulanic acid bound to penicillin-binding proteins only at a very high concentration in S. cacoi. This compound did not bind to those of S. olivaceus and S. clavuligerus. Penicillin-binding proteins in culture supernatant and cytoplasm and minimum inhibitory concentrations of these strains against benzylpenicillin were also examined.