Feeding greater quantities of protein and energy to neonatal calves was associated with a reduction in proliferative responses of T lymphocyte subsets to in vitro polyclonal stimulation. Feeding an intensified diet was also associated with altered in vitro expression of activation molecules, CD25, CD44, and CD62L. These data suggest that plane of nutrition during the neonatal period influences lymphocyte-activities essential for the development of a normal immune response.