Bovine luteal cells express class I and II major histocompatibility complex molecules and stimulate T lymphocyte proliferation in vitro. Proliferation of T lymphocytes is greater in cocultures of luteal cells and T lymphocytes collected following administration of a luteolytic dose of prostaglandin (PG) F_2α to the cow. Whether this results from changes in luteal cells that increase their ability to stimulate T lymphocyte proliferation or from changes in T lymphocytes that enhance their ability to respond to luteal cells is unclear. To determine which is the case, luteal cell-T lymphocyte cocultures were performed using luteal cells and T lymphocytes isolated from the same animals before and 8 h after administration of PGF_2α. In the presence of T lymphocytes collected before PGF_2α administration, luteal cells isolated after PGF_2α were more potent stimulators of T lymphocyte proliferation than were luteal cells collected before PGF_2α (P < 0.05). The effect of progesterone on luteal cell-stimulated T lymphocyte proliferation was also evaluated. Proliferation of T lymphocytes was greater (P < 0.05) in cultures containing the cytochrome P450 side-chain cleavage enzyme-inhibitor aminoglutethimide. Exogenous progesterone caused a dose-dependent inhibition of luteal cell-stimulated T lymphocyte proliferation (P < 0.05). Progesterone-receptor mRNA was undetectable in peripheral blood mononuclear cells collected before and after PGF_2α administration, indicating that the effect of progesterone was not mediated via progesterone receptors in lymphocytes. These results imply that specific changes in luteal cells in response to PGF_2α enhance the ability of these cells to stimulate T lymphocyte proliferation. These results also demonstrate that progesterone can suppress luteal cell-stimulated T lymphocyte proliferation.