Tall fescue (Festuca arundinacea Schreb.) is often infected by the endophytic fungus, Acremonium coenophialum (Morgan-Jones and Gams) The fungus produces ergo peptide alkaloids, especially ergovaline. Consumption of endophyte-infected (ED fescue forage by cattle decreases serum prolactin and average daily weight gains, which results in economic losses for producers. Anecdotal reports suggest cattle with fescue toxicosis may not respond to vaccination. Hyperprolactinemia decreases cell-mediated and humoral immune responses in mice. Therefore, steers grazing EI or endophyte-free (EF) fescue forages were vaccinated to assess humoral immune responses. Steers grazing EI fescue exhibited classical clinical signs of fescue toxicosis, and mounted humoral immune responses to vaccination, despite hyperprolactinemia. Lymphocyte proliferation responses to mitogens in mice fed EI diets were similar to mice fed EF diets. Production of interferon gamma and interleukin-4 was higher by splenocytes from mice fed EI diets, whereas interleukin-2 tended to be lower. Fescue toxicosis may stimulate T helper cell 2 subset of CD4⁺ T cells. The T_H2 subset may augment humoral immune responses to vaccination. / Ph. D.