Vitamin A and beta-carotene improved mammary health in dairy cows around dry off. To define possible mechanisms, cows were fed 1) 53,000 IU vitamin A, 2) 213,000 IU vitamin A, or 3) 53,000 IU vitamin A plus 400 mg beta-carotene/cow per d (n = 10/treatment) from 6 wk before to 2 wk after dry off. Blood polymorphonuclear neutrophil function (phagocytosis, kill, and chemotaxis) and lymphocyte proliferation were measured at wk -6, 0 (dry off), and 2. Concentrations of vitamin A in serum did not differ across vitamin treatments. beta-Carotene in serum was elevated in cows fed beta-carotene. Treatment did not influence phagocytosis or kill. Kill ability increased after dry off in all treatment groups, but phagocytosis tended to decrease after dry off in cows fed vitamin A only. Lymphocyte blastogenesis stimulated by concanavalin A on wk 2 for cows fed 53,000 IU vitamin A but did not vary in the other two groups. Lipopolysaccharide-stimulated blastogenesis peaked at wk 0 and then decreased to pretreatment values by wk 2 in cows fed 213,000 IU vitamin A. These data indicate lymphocyte function is influenced by vitamin A supplementation and that beta-carotene supplementation seems to exert a stabilizing effect on neutrophil and lymphocyte function during the period around dry off.