Thirty six lactating Holstein cows were selected based on identical parity, milk yield, body weight and body condition score to study the effect of supplemental β-carotene in the periparturient diet on plasma vitamin A (VA) and β-carotene concentrations and lymphocyte proliferation. The cows were randomly allotted into three groups and individually fed with one of three treatment diets: 1. control diet (basic diet + 150,000 IU VA/cow/day), 2. control diet + 300 mg β-carotene/ cow/day, and 3. control diet + 600 mg β-carotene/cow/day. The results showed that supplemental β-carotene increased the concentration of VA and β-carotene in plasma, and increased the lymphocyte proliferation with adequate VA in the diet during periparturient period (P<0.01).