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This study characterizes the expression of monocyte chemoattractant protein-1 (MCP-1) and the relative distribution of immune cell populations in the bovine corpus luteum throughout the estrous cycle. Immunodetectable MCP-1 was evident in corpora lutea of cows at Days 6, 12, and 18 postovulation (Day 0 = ovulation, n = 4 cows/stage). Day 6 corpora lutea contained minimal MCP-1 that was confined primarily to blood vessels. In contrast, relatively intense staining for MCP-1 was observed in corpora lutea from Days 12 and 18 postovulation. MCP-1 was again most evident in the cells of the vasculature, but it was also observed surrounding individual luteal cells, particularly by Day 18. An increase in immunohistochemical expression of MCP-1 on Days 12 and 18 postovulation corresponded with increases in MCP-1 mRNA and protein in corpora lutea as determined by Northern blot analysis and ELISA. Monocytes and macrophages were the most abundant immune cells detected in the bovine corpus luteum, followed by CD8+ and CD4+ T lymphocytes. In all instances, Day 6 corpora lutea contained fewer immune cells than corpora lutea from Days 12 and 18. In conclusion, increased expression of MCP-1 was accompanied by the accumulation of immune cells in the corpora lutea of cows during the latter half of the estrous cycle (Days 12-18 postovulation). These results support the hypothesis that MCP-1 promotes immune cell recruitment into the corpus luteum to facilitate luteal regression. These results also raise a provocative issue, however, concerning the recruitment of immune cells several days in advance of the onset of luteal regression.