B‐lymphocyte populations and the in vitro antibody producing activity of lymphocytes were measured to evaluate the lymphocyte function of periparturiment dairy cows. Changes in Blymphocyte populations were not significantly different in cows during this period Significantly decreased numbers of plaque‐forming cells were observed in lymphocytes from cows at parturition and 3 days after parturition compared to those in lymphocytes obtained 14 days after parturition.