On the membrane-bound penicillin-binding proteins (PBP) of Streptomyces cacaoi and S. clavuligerus, the kinetics of [14C]benzylpenicillin binding, the temperature sensitivity, the release of [14C]benzylpenicillin from the [14C]benzylpenicillin-PBP complexes and the changes of the PBP patterns during the growth cycle were examined. All the PBP in both strains, especially PBP in S. clavuligerus, had very low affinity for benzylpenicillin, comparing with other bacteria. As for the temperature sensitivity of the binding ability, all the PBP in S. cacaoi were highly sensitive to heat, while PBP-3 in S. clavuligerus retained the binding activity after incubation at 60 degrees C for 10 minutes. The release of [14C]benzylpenicillin from the complexes with PBP-1, PBP-2 in S. cacaoi and PBP-3 in S. clavuligerus was relatively fast initially. However, this soon reached a plateau and the complexes retained [14C]benzylpenicillin even after prolonged incubation. During the growth cycle, the PBP patterns in S. cacaoi did not change significantly. However, in S. clavuligerus, a band of molecular weight of about 120,000 daltons was observed only in the membrane fraction of early log phase, and PBP-1 (Mr=83,000) and PBP-2 (Mr=79,000) appeared only slightly in this phase.