Over a 2 1/2 year period 361 clinical isolates of clostridia, representing 28 species, were tested for susceptibility to commonly used antimicrobial agents. Penicillin resistant strains were tested for their ability to produce beta-lactamase: of the commonly isolated species only Clostridium beijerinckii/butyricum produced the enzyme. Cl. perfringens exhibited a low incidence of resistance to all of the agents tested. Cl. difficile showed a high degree of resistance to penicillin and clindamycin, whilst all strains were sensitive to vancomycin. Chloramphenicol and amoxycillin/clavulanic acid were found to be highly active against most clostridia; metronidazole was the only agent to which no resistance was demonstrable.