The effect of feeding lactic-acid bacteria on indices of functions of lymphocytes obtained from Peyer's patches, peripheral blood and spleen from inbred Wistar-Furth rats were studied. Rats were fed on purified diets supplemented with 350 g milk or yoghurt/kg diet for 4 weeks. At the end of the feeding period, immune cells from the three sites were isolated and proliferation, interferon-γ production and lymphocyte subset composition were studied. Rats consuming yoghurt had a greater in vitro proliferative response to yoghurt bacteria in the three lymphoid compartments, a greater interferon-γ production in response to bacteria and concanavalin A in Peyer's patches and spleen, and a greater number of Peyer's patches B lymphocytes than milk-fed rats. Macrophage and T lymphocyte proportions and lymphocyte subset composition in the three sites were unaffected by yoghurt. These results indicate that feeding live bacteria contained in yoghurt may interact with the intestinal immune system, and influence the systemic immune system.