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The following results were obtained from the comparison of cefoperazone (CPZ) with cefazolin (CEZ), cephalothin (CET) and cefoxitin (CFX) in respect to their antibacterial action against anaerobic bacteria: 1) CPZ showed an antibacterial spectrum which was comparable to those of the known antibiotics, CEZ, CET and CFX. CPZ was particularly effective against anaerobic Gram-positive bacteria. 2) CPZ showed a distribution of susceptibility against clinically isolated strains which was comparable to those of CEZ, CET and CFX, CPZ was slightly inferior to CFX against B. fragilis and B. thetaiotaomicron, but was somewhat superior to any of CEZ, CET and CFX against anaerobic Gram-positive rods and B. distasonis. 3) The susceptibility of B. fragilis and F. necrophorum to CPZ decreased with an increase in the amount of inoculation, while C. perfringens and P. asaccharolyticus did not show any difference in susceptibility with the amount of inoculation. 4) The values of MIC and MBC of CPZ showed good coincidence with each other, thereby testifying its superior bactericidal action. 5) Natural resistant mutants were not obtained. 6) According to the examination of resistance in a test tube, P. variabilis did not show any appreciable increase in resistance, but F. varium increased its resistance rapidly. 7) CPZ was inferior only to CFX in stability against the beta-lactamase of B. fragilis. All of CEZ, CET and CMD lacked stability against the beta-lactamase of B. fragilis. 8) CPZ was found effective for the treatment of experimental subcutaneous abscess in mice caused by F. necrophorum.