The adjuvant effect of kefir fermented milk on the mucosal and systemic immune systems was examined in young (6 mo old) and old (26 mo old) rats. Kefir-fed rats consisted of young or old rats consuming kefir-fermented milk ad libitum on a daily basis in addition to the standard diet, for 28 d. Control rats consumed only the standard diet. The rats were immunized intraduodenally with cholera toxin (CT) on d 7 and 21 and killed on d 28. The nonspecific serum immunoglobulin (Ig)A titers in kefir-fed and control rats did not differ in either age group. The serum anti-CT IgA antibody concentrations were significantly higher in the kefir-fed young rats compared with their age-matched controls (+86%, P: < or = 0.05). This difference was associated with enhanced in vitro antibody secretion by cultured lymphocytes isolated from the Peyer's patches and the intestinal lamina propria (+180%, P: < or = 0.05). These enhanced responses were found only in the young rats. However, the nonspecific serum IgG titer was higher (>120%, P: < or = 0.05) and the anti-CT IgG titer was lower (-80%, P: < or = 0.05), in both young and old kefir-fed rats compared with their respective controls. Nevertheless, these results demonstrate that a kefir-supplemented diet affects the intestinal mucosal and systemic immune responses to intraduodenal CT differently in young and old rats. Most importantly, our data suggest that orally administered kefir enhances the specific intestinal mucosal immune response against CT in young adult, but not in senescent rats.