Vancomycin pharmacokinetics were studied in four patients with peritonitis undergoing chronic intermittent peritoneal dialysis. Serum levels exceeding 4.0 micrograms/ml were maintained for 8 and 13 days after a single 1-g intravenous dose. Vancomycin serum concentrations measured before, during, and upon completion of dialysis revealed no appreciable decline. Peritoneal fluid concentrations in two patients exceeded 4.0 micrograms/ml for more than 12 days.