We studied the infiltration of different subsets of immune system cells in the ovarian parenchyma of Egyptian buffaloes during follicular and luteal phases of the estrous cycle. All subsets of leukocytes infiltrated significantly more into corpora lutea (CL) than into Graafian follicles (GF) (P < 0.01) except for plasma cells that were abundant in the GF but not observed in the CL. The number of macrophages, lymphocytes, neutrophils and eosinophils were significantly greater in mature CL than in corpora hemorrhagica (CH) or regressing CL. Moreover, the regressing CL showed significantly more macrophages, lymphocytes and neutrophils than the CH. Large antral follicles were infiltrated with larger number of leukocytes than growing preantral atretic follicles. Macrophages and neutrophils observed in large antral follicles were significantly more abundant in the theca externa than the theca interna (P < 0.01). Only plasma cells were significantly greater in number in the theca intema (P < 0.01). Leukocytes infiltrated significantly more into large mature follicles than large, growing, preantral atretic follicles (P < 0.01). Results of this study reveal the calling of leukocytes in a significant numbers inside the ovarian tissue of buffaloes around the time of ovulation and at luteolysis. It is possible that leukocytes with their powerful bioactive cytokines (IL-1, TNFalpha, GM-CSF, and INF-gamma) may assist in ovarian functions such as ovulation and luteolysis.