The efficacy of cephalothin sodium, added to dialysis fluids, as prophylaxis against infection during peritoneal dialysis was tested in a random double-blind study of 95 dialyses performed in 36 patients. Although the two cases of clinical peritonitis observed occurred in the placebo group, this observation was not statistically significant. The number of positive cultures obtained from the dialysate returns, however, was significantly greater in the placebo group. The duration of the dialysis did not correlate with the finding of bacteria in the dialysate returns or with the development of clinical peritonitis.