This study investigated the effect of a duodenal infusion of a C18:3 free fatty acid on the immune function of lactating dairy cows. Four primiparous Chinese Holstein cows fitted with duodenal cannulas received 0, 100, 200, 300, and 400 g/d of alinolenic acid (LNA) in a two-treatment crossover design. Blood was collected and serum IgA, IgG, IgM, prostaglandin E2 (PGE2) and Th1/Th2 cytokines were determined. Results showed that increasing the supply of LNA to the small intestine of dairy cows linearly increased serum IgG and quadratically enhanced interferon-g (p 5 0.05), whereas the concentrations of PGE2 declined linearly (p 5 0.05) and those of interleukin (IL)-4 tended to decrease (p ? 0.08). No difference was observed in serum IgA, IgM or other cytokines, such as IL-2, IL-6 and IL-10. This study demonstrated that in dairy cows, a post-ruminal infusion of high doses of LNA has immunomodulatory effects, possibly associated with a predisposition to a Th1-type response.