Impaired neutrophil and lymphocyte function during the periparturient period is a contributing factor to the high incidence of infectious disease observed in the periparturient cow. Using 8 multiparous Jersey cows, we demonstrated that populations of T cells, especially T helper cells and gd T cells declined during the periparturient period coincident with reported losses of immune cell function;We hypothesized that milk production may be an important immunosuppressive factor. To test this hypothesis, we used 10 mastectomized and 8 intact multiparous Jersey cows and examined phenotype of peripheral blood mononuclear cells (PBMC), adhesion molecule expression and myeloperoxidase activity in neutrophils, and plasma level of steroid hormones. In intact cows, all T cell subset populations (CD3, CD4, CD8 and gd T cell receptor positive cells) showed significant declines toward parturition, and monocyte percentage increased significantly at parturition. These changes were significantly different from mastectomized cows. Mastectomy eliminated almost all changes in leukocyte subset proportions seen around parturition;Expression of leukocyte adhesion molecules was different in intact and mastectomized cows. Expression of b 2-integrins in intact cows was highest at parturition, and this expression was greater in intact cows than in mastectomized cows throughout the study. L-selectin expression exhibited a sudden decrease at parturition with recovery within a day after parturition in both intact and mastectomized cows. The ability of neutrophils to kill microbes as assessed by neutrophil myeloperoxidase activity decreased before parturition in both groups. While there was a quick recovery of neutrophil myeloperoxidase activity in mastectomized cows, there was no recovery in intact cows after parturition throughout the study which lasted until d 20 post partum. Milk production seems to exacerbate periparturient immunosuppression, especially with regard to recovery of neutrophil myeloperoxidase activity;Steroid hormone (progesterone, estrone, estradiol, and cortisol) analysis showed that mastectomy did not decrease the level of steroid hormones but rather increased estrogen level. Increase in steroid hormones do not seem to contribute to the periparturient immunosuppression in PBMC, although they may have certain effects on neutrophil function and L-selectin expression, especially before parturition.