AbstractXanthan gum (XG), a microbial polysaccharide produced extracellularly by fermentation ofXanthomonas campestris, has unique physical properties. We studied the effects of XG on murine lymphocytesin vitroand found that XG induced both a significant increase of DNA synthesis in mouse splenic B cells and thymocytes as well as polyclonal IgM and IgG antibody responses in B cells. XG‐activated thymocytes, however, did not display helper or suppressor functions. XG was almost as effective in inducing polyclonal antibody responses as lipopolysaccharide (LPS) in murine systems. Hamster spleen cells, however, were weakly triggered to nonspecific antibody production by XG but they were not triggered at all by LPS. Spleen cells from normal neonatal mice and from adult CBA/N mice, a strain which possesses an X‐linked defect affecting B cell differentiation, responded relatively well to XG and LPS, suggesting that XG can stimulate immature B cells as well as LPS does. It was found that XG activated B cells in the relative absence of T cells and macrophages. Spleen cells from LPS‐nonresponder C3H/HeJ mice and seven other mouse strains were stimulated to polyclonal antibody production by XG. In contrast, spleen cells from C57BL/10 mice were unresponsive or only slightly responsive to XG, but fully responsive t