The intraperitoneal instillation of noxythiolin in the treatment of peritonitis is widespread in clinical practice despite contradictory evidence as to its efficacy. In this light the value of noxythiolin was reappraised by studying its effect in guinea-pigs and mice with induced bacterial peritonitis. Treatment with a 1% solution of noxythiolin reduced the mortality rate of mice by 14% (P less than 0.1). The guinea-pig model proved unreliable giving inconsistent mortality rates throughout. Further studies are required to determine the optimum dose and concentration of noxythiolin while the search for more effective intraperitoneal antiseptics should continue.