In a randomised, coordinated six-centre study 163 patients with serious systemic infections received treatment with either imipenem (N-formimidoyl thienamycin) plus cilastatin, an inhibitor of its renal metabolism (77, I/C group) or gentamicin and clindamycin (86, G/C group); 56 and 62, respectively, were evaluable. Significantly more G/C than I/C patients failed to respond to treatment (9 vs 2) and 1 G/C patient died of infection. The frequency of elimination of causative pathogens was higher in the I/C group (88% vs 77%). Clinical and biochemical adverse reactions were less common in the I/C than the G/C group. Treatment had to be discontinued because of adverse reactions in 3 I/C patients and in 7 G/C patients. Clinical superinfections were noted in 1 I/C and in 2 G/C patients. Thrombophlebitis was significantly more common in the I/C group. In terms of clinical and bacteriological efficacy and safety, the I/C combination was superior to gentamicin/clindamycin.