Immune cells, which constitute a significant cell mass in the corpora lutea (CLs), are considered to play critical roles in luteolysis, but the details are not fully understood. We histochemically investigated the changes in distribution and cell density of macrophages and T lymphocytes and in tumor necrosis factor (TNF)-α and interferon (IFN)-γ, which can induce apoptosis in the luteal cells in murine CLs during luteal regression. No macrophages or T lymphocytes were observed in functionally healthy CLs. Abundant macrophages and increasing T lymphocytes were demonstrated in CLs at the functional regression stage (early stage of regression). At the structural regression stage (late stage of regression), abundant T lymphocytes but no macrophages were demonstrated in the CLs. A moderate amount of TNF-α was detected in all CLs at all stages. No IFN-γ was detected in either healthy or early regressing CLs, but a large amount of IFN-γ was detected at the late regression stage. Moreover, in cultured luteal cells, reactivity against Fas-ligand (FasL) was caused by pretreatment with TNF-α and IFN-γ and apoptosis was induced by FasL treatment. These findings support the hypothesis that macrophages initiate T lymphocyte aggregation at the early stage of luteal regression, and then T lymphocytes induce apoptosis on luteal cells, which in turn develop sensitivity against FasL by TNF-α and IFN-γ. J. Exp. Zool. 296A:152–159, 2003. © 2003 Wiley-Liss, Inc.