Cultures of Yersinia enterocolitica grown at 22 degrees C produced beta-lactamases, whereas cultures grown at 37 degrees C produced these enzymes much less effectively. Both dicloxacillin and clavulanic acid inhibited the beta-lactamase activity of bacterial crude extracts and potentiated the activity of penicillin G or cephalothin against 14 Y. enterocolitica strains. It appeared that the beta-lactamase activity present in Y. enterocolitica cells grown at 37 degrees C was great enough to play a role in bacterial resistance to beta-lactam antibiotics, since combining penicillin G or cephalothin with clavulanic acid or dicloxacillin resulted in synergistic activity against cultures grown at 37 degrees C that was equal to or greater than the activity against cultures grown at 22 degrees C.