Short-chain fatty acids (SCFA) are produced by fermentation of water-soluble fiber by anaerobic bacteria in the large bowel. Fiber-rich diets decrease the risk of developing inflammatory bowel disease (IBD) and butyrate enemas are effective as a therapy in some patients. Crohn's disease, one form of IBD, appears to involve an exagerated T helper-1 (Th1) lymphocyte phenotype, characterised by production of interleukin (IL)-2 and interferon (IFN)-γ, that drives the inflammation. To examine whether SCFA influence pro- and anti-inflammatory cytokine production, rat mesenteric lymph node lymphocytes were cultured in the presence of acetate (10 mM), butyrate (1.5 mM) or propionate (2 mM) and the production of cytokines in response to concanavalin A determined. Butyrate, but not acetate or propionate, inhibited lymphocyte proliferation and IL-2 production. Acetate and propionate were able to partly prevent the inhibitory effect of butyrate on IL-2 production. Acetate and propionate increased IFN-γ production, whereas butyrate inhibited it. Acetate and propionate in combination were able to prevent the inhibitory effect of butyrate on IFN-γ production. IL-4 was not detected in any cultures. Acetate and propionate increased IL-10 production, which was not affected by butyrate. It is concluded that butyrate significantly inhibits Th1-type responses and that this might explain the therapeutic effect of butyrate in IBD patients. Acetate and propionate have less marked modulatory actions, and in some cases have effects that oppose those of butyrate. A combination of the three SCFA causes a shift in the T helper lymphocyte phenotype towards a more anti-inflammatory phenotype and this might explain the protective effects of fiber.