Leukocytes including monocyte/macrophages, granulocytes, and T lymphocytes were localized in the corpus luteum (CL) of pregnant and pseudopregnant rats using monoclonal antibodies reactive with lineage-specific antigens. Neutrophilic granulocytes and monocytes/macrophages were found to be the most abundant leukocyte populations in the CL during both pregnancy and pseudopregnancy. The density of neutrophilic granulocytes (MCA 149-reactive cells) increased approximately 2-fold after mating, to peak on Day 9 of pseudopregnancy (214 ± 13 positive cells/0.125 mm² grid area) and Day 10 of pregnancy (216 ± 12/grid area), and then declined in later stages of CL life. In pregnant rats, monocytes/macrophages positive for the monoclonal antibody ED¹ were most numerous during early CL life when they were approximately 6-fold more plentiful than at luteolysis (153 ± 14/grid area at Day 5 vs. 25 ± 2 at 2 days postpartum). In pseudopregnant rats, the density of ED1-positive cells declined approximately 5-fold during the life span of the CL (124 ± 17/grid area at Day 2 vs. 20 ± 2 at Day 13) prior to a second, somewhat lesser peak at luteal regression (84 ± 12 at Day 15). Fewer monocyte/macrophages within the CL were found to express antigen reactive with the monoclonal antibody ED₂, which is characteristically a marker for tissue-macrophages (7% of the density of ED₁-positive cells at Day 5 in the pregnant group and 13% at Day 2 in the pseudopregnant group); and while there was an approximately 60% reduction in the number of ED₂-positive cells during pregnancy, these cells were found not to fluctuate significantly in number over the course of CL life in pseudopregnant animals. In contrast, the number of major histocompatibility complex class II (OX6)-reactive cells increased approximately 4.5-fold and 2.5-fold in pseudopregnant and pregnant rats, respectively, suggesting that luteal regression is accompanied by enhanced macrophage activation. T lymphocytes (OX52- positive cells) were found to have a consistently sparse distribution within the CL (1% of the density of ED₁,-positive cells at Day 5 in the pregnant group and 2% at Day 2 in the pseudopregnant group). These data indicate that leukocytes of the myeloid lineages comprise substantial and dynamic populations within the rat CL, and suggest that these cells may have roles both in regulating steroidogenesis and in the tissue remodeling that accompanies CL development and demise during pregnancy.