Interleukin-2 (IL-2) was one of the first hormones recognized in the immune system as having a role in T-cell growth and its secretion in the systemic circulation is increased during disease (for review see: Blecha, 1991; Minami et al., 1993). Recent studies suggest that IL-2 may be used as adjunct therapy with existing vaccines (Campos et al., 1993) and mastitis antibiotics (Daley et al., 1992) to improve therapeutic efficiency as well as a general immunostimulant when drying off dairy cows (Nickerson et al., 1992). Whether these treatments will alter reproductive functions is unknown. In spite of the fact that other cytokines such as tumor necrosis factor-α (TNFα) seem to affect ovarian cell function (Brannstrom and Norman, 1993), the role of IL-2 in ovarian steroidogenesis is practically unexplored. Hence, the objectives of our studies were to determine if IL-2 can directly affect ovarian function in cattle by evaluating the effects of IL-2 on steroidogenesis of granulosa and thecal cells in vitro. These studies may also provide useful insights as to the mechanism whereby disease alters reproductive efficiency.