The effects of in vitro supplementation of beta-carotene, retinol, and retinoic acid on phagocyte function during the peripartum period were assessed. Blood was collected at wk -4, -1, 0 (calving), 1, and 4; mammary secretions were collected at wk -1, 0, 1, and 4 from 14 Holstein cows for the isolation of phagocytic cells. Blood polymorphonuclear leukocytes and mammary macrophages and polymorphonuclear leukocytes (phagocytic cells) were assayed for phagocytic and intracellular kill abilities of Staphylococcus aureus in the presence of beta-carotene and retinol at 10(-8) and 10(-7) M and retinoic acid at 10(-9) and 10(-8) M. Phagocytosis by blood or milk phagocytic cells was not influenced by beta-carotene. However, beta-carotene enhanced kill by blood and milk phagocytic cells during certain prepartum and post-partum periods. In contrast to beta-carotene, retinol and retinoic acid either had no effect or suppressed phagocytosis and kill. These results are interpreted to suggest a mechanism by which beta-carotene affords the mammary gland protection against infection, i.e., through enhanced intracellular kill by phagocytes.