We compared the effectiveness and incidence of nephrotoxicity of ampicillin-tobramycin and cefotaxime in 73 cirrhotics who had severe bacterial infection. Most of these patients had spontaneous peritonitis and/or bacteremia. Patients were randomly allocated into two groups. Group I included 36 patients treated with ampicillin-tobramycin and Group II comprised 37 patients treated with cefotaxime. Patients from both groups were similar with respect to clinical data, standard liver and renal function tests, types of infection and isolated organisms. Ninety-two per cent of bacteria isolated in Group I and 98% of those isolated in Group II were susceptible in vitro to ampicillin-tobramycin and to cefotaxime, respectively. Ampicillin-tobramycin cured the infection in 56% of Group I patients, and cefotaxime in 85% of Group II patients (p less than 0.02). Five patients treated with ampicillin-tobramycin, and none treated with cefotaxime developed superinfections (p = 0.024). Nephrotoxicity (impairment of renal function associated with an increase of urinary beta 2-microglobulin to over 2,000 micrograms per liter) occurred in two patients in Group I and none in Group II. These results suggest that broad-spectrum cephalosporins should be considered as first choice antibiotics in cirrhotic patients with severe infections.