Immune reactivity towards the bacterial intestinal flora plays an important part in the pathogenesis of inflammatory bowel disease. Disease activity can be positively influenced by the administration of living probiotic bacteria. We investigated the effect of soluble bacterial antigens extracted from Escherichia coli (strain Laves) on the disease activity of murine colitis. C3H.IL-10-/- and BALB/c mice with dextran sulphate sodium-induced colitis were treated with either a bacterial lysate from E. coli or with a placebo. Mice were monitored and inflammation was assessed by histological scoring, analysis of fecal IL-1beta and measurement of cytokine production by ELISA. T cell proliferation was quantified by 3H-thymidine incorporation. Clinically and histologically, bacterial-lysate-treated mice revealed significantly (P < 0.05) fewer signs of colitis than placebo-treated mice. Fecal IL-1beta and mucosal TNF-alpha and IFN-gamma concentrations were significantly lower (P < 0.05) in verum-treated mice than in the placebo group. Furthermore, lymphocyte proliferation after stimulation with lipopolysaccharide or caecal bacterial antigen was significantly (P < 0.05) reduced in verum-treated mice. The use of E. coli lysate is effective in the amelioration of murine colitis. This effect may be due to a decreased Th1 reaction and to an induction of tolerance against bacterial antigens.