The objective of this study was to evaluate the effect of HMB on selected indicators of immunity in calves. The experiment was performed on 14 calves aged 30 +/- 2 days, divided into two equal groups of control (group I) and experimental (group II) animals. The feed administered to experimental group calves was supplemented with HMB at 40 mg/kg BW, whereas control calves were administered standard farm-made feed without supplementation. Blood was sampled from the jugular vein immediately before the experiment (day 0) and on experimental days 15, 30 and 60 to determine the following parameters of immunity: proliferative response of LPS- and ConA-stimulated lymphocytes (MTT), respiratory burst activity (RBA) and potential killing activity (PKA) of phagocytes. The results revealed a significant increase in RBA and MTT values in calves administered HMB in comparison with the control group throughout the experiment. In the group of animals receiving HMB, an increase in PKA values was noted only on day 30.