Muscular injury caused by cefoperazone (CPZ) was compared with that of cefazolin (CEZ), cephaloridine (CER) or cephalothin (CET) in adult and juvenile male rabbits. These drugs were dissolved by the way of clinical use and were injected singly into the muscular vastus lateralis. Then, the degree of muscular injury at the time of 48 hours and 7 days after the injection was judged from muscle ratio, gross local observation and histological observations. The degree of muscular injury caused by these drugs was compared also with that of saline 075% or 6% acetic acid. The following results were obtained: There was no difference among CPZ, CEZ and CER on the degree of muscular injury and these changes were almost equal to those of 0.75% acetic acid, however severer than that of saline. While, muscular injury caused by CET was severer than that of CPZ, CEZ, CER or 0.75% acetic acid, but milder than 6% acetic acid. The inflammatory reaction against these drugs was almost similar in adult and juvenile rabbits.