Imipenem-cilastatin was compared, in two multicentre clinical trials, with cefazolin or cephalothin, in their efficacy, safety, and tolerance. Safety and tolerance for imipenem-cilastatin was similar to that of these two cephalosporins. There were more transient liver function test changes in the imipenem-cilastatin group, but the frequency was similar to that for beta-lactams in general. There were similar numbers of cases of transient neutropenia in each group. Clinical side effects were few for the treatment groups and all drugs were well tolerated.