β-methyl-βbutyrate (HMB) has been found to be markedly more effective for activating the immune function of T lymphocytes of mammals than α-ketoisocaproate (KIC). For activation of the T lymphocytes, β-methyl-butyric acid or an edible water-soluble salt thereof is administered to the mammal by a route through which the HMB enters the blood of the mammal. The amount administered is sufficient for effective enhancement of the blastogenesis of their T lymphocytes. The method is adapted for use with domestic mammals, including particularly cattle, sheep, and swine. HMB can also be used with humans as an immune system stimulant.