Thirty strains of Cl. perfringens isolated from clinical material were found to be inhibited in vitro by 3 μg/ml or less of cephalothin and cephaloridine, 12 μg/ml or less of cephalexin and 6 μg/ml or less of cephaloglycin, as determined with the agar and microtiter broth dilution techniques. Cephaloridine appeared somewhat more active than cephalothin and cephaloglycin, while cephalexin appeared to be slightly less active than the former 3 cephalosporins, on a weight-for-weight basis. With few exceptions, the MBC’s of all 4 cephalosporins either coincided with or were twofold higher than the corresponding M