Four common Lactobacillus strains were screened for their effects on proliferation of mouse splenic lymphocytes. Mice received perorally 10(9) viable bacteria kg(-1) body weight for 7 days. Lactobacillus acidophilus treatment enhanced ex vivo basal proliferation (by 43%) and B-cell response at suboptimal and optimal concentrations of lipopolysaccharide (LPS) (by 27-28%). Conversely, Lactobacillus casei, Lactobacillus gasseri and Lactobacillus rhamnosus inhibited both basal proliferation (by 14-51%) and mitogen-stimulated lymphoproliferation, particularly at supra-optimal concentrations of concanavalin A (by 43-68%) and LPS (by 23-62%). Therefore, these Lactobacillus strains demonstrate strain-specific effects on B- and T-cells and may also alter the splenocyte sensitivity to the cytotoxic effects of mitogens.