The neonatal calf has a heightened susceptibility to infectious disease. Traditional calf-rearing programs limit nutrient intake from milk or milk replacer during the first few weeks of life to promote dry-feed intake and early weaning. Dramatic improvements in growth performance achieved by feeding increased amounts of milk replacer with more protein is hypothesized to be associated with enhanced immune function and an increased resistance of the calf to infectious disease. Objectives of this dissertation were to characterize more extensively immune responses of the neonatal calf and to determine if intensified nutrition enhances immune function of milk replacer-fed calves. T-cell subsets from 1-wk-old calves showed decreased proliferative activity, a delayed increase in CD25 expression, and no demonstrable increase in CD44 expression or decrease in CD62L expression when compared with the mitogen-induced responsiveness of cells from steers. Mitogen-induced proliferation and expression of activation antigens by T cells from 8-wk-old, standard-fed calves, however, were similar to responses of cells from steers, indicating that T cell function during the neonatal period matures rapidly. Intensified nutrition during the neonatal period was associated with alterations in the responsiveness of T cell subsets to mitogenic stimulation, characterized by decreased proliferation, decreased expression of activation antigens (i.e., CD25 and CD44), decreased interferon-gamma secretion, and increased nitric oxide production. These results suggest that animal maturity and neonatal nutrition influence functional activities of T lymphocyte subsets essential in the development of cell-mediated immunity. Feeding a milk replacer (30% crude protein/20% fat) at different rates to achieve no growth, low growth, and high growth did not affect antigen-specific recall responses of cells from vaccinated calves. These results suggest protein-energy malnutrition in the absence of weight loss does not affect adaptive immune responses of calves. Cells from high-growth rate calves had increased nitric oxide production and decreased viability. These alterations in cell function may have deleterious effects on resistance to infectious disease.