Cefoperazone is a third-generation cephalosporin that is active against a broad spectrum of gram-positive and gram-negative bacteria. We added this antibiotic to peritoneal dialysis solution at a concentration of 62.5 mg/L to treat peritonitis in six continuous ambulatory peritoneal dialysis (CAPD) patients. Serum drug concentrations were obtained at 0, 0.5, 1, 2, 4, and 24 h after instituting antibiotic therapy. Rapid uptake by blood of the antibiotic across the peritoneal membrane occurred when the latter was inflamed. Adequate bactericidal serum levels for many bacteria were obtained in less than 4 h. Cefoperazone effectively eradicated peritonitis in all patients.