Protection against Listeria monocytogenes and Escherichia coli in mice was enhanced by an intraperitoneal (i.p.) administration of polysaccharide "RBS". Peritoneal macrophages from mice administered i.p. with 30 mg/kg doses of RBS 4 days earlier exhibited increased scavenger functions as assessed by in vivo phagocytosis, in vitro intracellular killing and generation of superoxide anion. When cytokine production of the macrophages was assessed by biological assay and Northern blotting analysis, interleukin (IL)-1 production and IL-1 alpha gene expression were significantly increased in macrophages from RBS-treated mice. On the other hand, tumor necrosis factor (TNF) alpha gene was expressed in macrophages from RBS-treated mice at a much reduced level as compared with those in mice treated i.p. with Corynebacterium parvum on 4 days earlier. In correlation with expression of TNF gene in the macrophages, RBS-treated mice were less susceptible to the lethal toxicity of LPS than C.parvum-treated mice. In RBS-treated mice, in vivo elimination of bacteria was enhanced at the early phase of infection with L.monocytogenes or E.coli, resulting in augmentation of host defense against these bacterial infection. These results suggest that adequately enhanced activities of macrophages acting as scavenger phagocytes play important roles in the enhanced resistance against bacteria in mice treated i.p. with RBS.