SummaryLymphocyte blastogenic response, B‐lymphocyte population and antibody‐producing activity of lymphocytes were determined to evaluate the lymphocyte function in neonatal calves during the first 4 weeks of life. The mean percentage of B‐lymphocytes ranged from 10.2 to 12.5% during the first 14 days of life and from 15.3 to 17.5% in calves from the day 21 to day 28 after birth. The absolute number of B‐lymphocytes increased significantly (P<0.05) from 370/μl at birth to 736/μl on day 28 after birth. The mean stimulation index of blastogenic response, measured by fluorometric assay, ranged from 5.75 to 6.61 with Con A, from 5.29 to 5.98 with PHA and from 1.89 to 2.50 with PWM. The mean (± S. D.) number of plaque forming cells ranged from 22.0 (± 12.0) to 24.7 (± 9.2) in cultured lymphocytes from 5 calves at birth to 14 days after birth and their levels increased markedly from 137.8 (± 88.3) to 162.0 (± 57.8) in lymphocytes from 20 days to 28 days after birth. The present study showed that antibody‐producing activity of lymphocytes is lower in calves within 3 weeks after birth compared to that of calves 3 weeks after birth, indicating that neonatal calf lymphocytes have a low antibody‐producing activity at least up to 1