Forty patients with severe bacterial infections due to micro-organisms known or presumed to be sensitive to both study antibiotics, were randomized to receive either imipenem with cilastatin, 500 mg/500 mg iv tid (20 patients), or cefotaxime, 2 g iv tid (20 patients). The types of infections observed were equally distributed between the 2 groups, and consisted of 18 complicated urinary tract infections, 9 pneumonia, 7 bone and soft tissue infections, 4 septicaemia of unknown origin and 2 intravenous-catheter-related septicaemia. In the imipenem group, 12 patients were bacteraemic, compared to 10 in the cefotaxime group. The micro-organisms observed were evenly distributed with Escherichia coli (22 cases) and Klebsiella pneumoniae (6 cases) being the most frequent. Sixteen patients were cured in the imipenem group and 15 in the cefotaxime group, while 2 and 2 improved, 2 and 1 relapsed and 0 and 2 did not respond to the therapy, respectively. In the imipenem group, no clinical side effects were observed while 5 patients had mild reactions in the cefotaxime group (2 fever, 1 skin rash, 1 oral candidiasis and 1 diarrhoea). Imipenem thus appeared as effective and well tolerated as cefotaxime in the treatment of severely infected patients.