Mice challenged with Escherichia coli or Staphylococcus aureus were protected against lethal peritonitis by the intravenous administration of 10 micrograms of poly-beta 1-6-glucotriosyl-beta 1-3-glucopyranose (PGG) glucan per animal 4 to 6 h prior to bacterial challenge. Subsequent studies with the rat model for intra-abdominal sepsis indicated that intramuscular doses of 10 to 100 micrograms per animal 24 and 4 h prior to surgical implantation of the bacterial inoculum reduced the early mortality associated with the peritonitis phase of this experimental disease process. Quantitative cultures of blood obtained from challenged rats showed that significantly fewer organisms were present in the blood of PGG glucan-treated animals than in that of untreated animals. Quantitative studies of leukocytes of rats and mice following a single injection of PGG glucan showed a modest transient increase in the total leukocyte count. The possible mechanisms by which protection occurs in the animal model system are discussed.