The pharmacokinetics of cefamandole nafate, a new parenteral cephalosporin derivative, were evaluated in 11 patients with chronic renal failure (creatinine clearance less than 5 ml/min), including five patients during hemodialysis, four patients during routine peritoneal dialysis, and two patients during the interdialytic period. Peak serum levels of cefamandole were comparable to those observed in patients with normal renal function. Clearance of the drug during the interdialytic period and during hemodialysis and peritoneal dialysis was minimal, with a resultant significant prolongation of serum half-life. The nondialyzability of cefamandole is in contrast with reported studies of cephalothin, where significant reduction of the serum half-life was achieved during hemodialysis but not peritoneal dialysis. The concentration of cefamandole in the peritoneal dialysate after parenteral administration was observed to be bactericidal for many gram-negative pathogens and, with the exception of Streptococcus faecalis, most gram-positive organisms found in bacterial peritonitis in patients with severe renal failure. The present data suggest that if stable bactericidal serum levels of cefamandole are to be maintained during hemodialysis and peritoneal dialysis, a parenteral loading dose must be administered followed by one-half the loading dose every half-life.