Purified bovine milk proteins that were added to cultures of murine spleen cells significantly increased cell proliferation and production of immunoglobulin M. Casein and a whey mixture consisting of alpha-lactalbumin, bovine serum albumin, bovine gamma globulin, and beta-lactoglobulin (beta-LG) were stimulatory. Of the three beta-LG preparations that are commercially available (beta-LG containing variants A and B, purified variant A, and purified variant B), the unseparated mixture containing both the A and B variants showed the most immunomodulatory activity. Both alkaline treatment and trypsin digestion of the beta-LG preparation markedly reduced its effectiveness. Polymyxin B, while greatly diminishing the stimulatory effect of lipopolysaccharide, had no significant effect on either the enhancement of cell proliferation or the enhancement of immunoglobulin production by beta-LG. In the presence of S-(n-butyl)-homocysteine sulfoxamine, the stimulatory effect of beta-LG on cell proliferation and IgM production in vitro was markedly reduced.