Tobramycin in combination with clindamycin or lincomycin were used as systemic antibiotics in the treatment of 20 consecutive patients with septic peritonitis or intraabdominal sepsis, 10 of which were in septic shock. Doses were: tobramycin 1.5 mg/kg body weight every 8 hours, with prolonged dosage interval in patients with reduced renal function, clindamycin 0.9 g every 8 hours and lincomycin 1.2 g every 8 hours. Therapy was monitored by means of tobramycin serum concentration determinations and renal function tests. Eventual cure of the infection was obtained in 19 patients. In 2 of these, the effects of the antibiotics were doubtful. Side effects were observed on 8 occasions: One patient had a slight and temporary subjective hearing loss, coinciding with raised trough levels of tobramycin. Diarrhoea occurred in 3 cases and skin reactions in 3 cases. Superinfection with Candida albicans fungemia occurred in one patient. From the overall results it is concluded that the antibiotic regimen is of value in serious life-threatening infections. Although the tobramycin dose was higher than customarily used in Scandinavia at the time, 0 hour and 1 hour serum concentrations remained stable during therapy in patients whose renal function was normal at onset of therapy. Serum creatinine (S-Cr) levels in these patients were also essentially unchanged. Temporary reductions in osmolality (Osm) ratio Osm-urine/Osm-serum occurred in 11 patients despite normal S-Cr, but it was hard to attribute these impairments of renal function to tobramycin specifically. It was also doubtful whether tobramycin further aggravated renal function in those patients where it was impaired at onset of therapy. Thus, no conclusive evidence of clinically important tobramycin-induced nephrotoxicity were found. We suggest that the dosage schedule of tobramycin used in this study is applied when treating serious intraabdominal infections.