Effects of increased protein and energy provided by an intensified milk replacer on the antigen-specific, cell-mediated immune response of the neonatal calf were examined. Calves were fed a standard (0.45 kg/day of a 20% crude protein, 20% fat milk replacer; n=11) or intensified (1.14 kg/day of a 28% crude protein, 20% fat milk replacer; n=11) diet from 0 to 6 weeks of age. All calves were vaccinated with Mycobacterium bovis bacillus Calmette-Guerin (BCG) at 1 week of age. The daily weight gain of intensified-diet calves (0.62 kg/day) was greater than the weight gain of standard-diet calves (0.29 kg/day). Liver, kidney, heart, thymus, and subcervical lymph nodes from intensified-diet calves were heavier than the same organs from standard-diet calves. Flow cytometric analysis of peripheral blood mononuclear cell (PBMC) populations indicated that CD4+ cells, gamma delta TCR+ cells, and monocyte percentages, although unaffected by diet during the first 5 weeks of the study, were higher in intensified-diet calves at week 6. The decline in gamma delta TCR+ cell percentages and increase in B cell percentages with increasing age seen in all calves are characteristic of the maturing immune system of the calf. CD8+ T cell or B cell percentages were not affected by diet. In intensified-diet calves, percentages of CD4+ expressing interleukin-2 receptor increased and percentages of gamma delta TCR+ cells expressing interleukin-2 receptor decreased with time. The same populations in standard-diet calves did not change with time. Percentages of CD4+ and CD8+ T cells, and B cells expressing MHC class II antigen, were unaffected by diet or age. Although mitogen-induced interferon (IFN)-gamma and nitric oxide (NO) secretion increased with age for all calves, PBMC from intensified-diet calves produced less IFN-gamma and more NO than did cells from standard-diet calves at week 6 of the study. Antigen-induced secretion of IFN-gamma and NO also increased with age but was unaffected by diet. Antigen-elicited delayed-type hypersensitivity was unaffected by diet, suggesting increased dietary protein and energy did not alter adaptive immunity in vivo. Overall, these results suggest that feeding calves a commercially available, intensified milk replacer affects minimally the composition and functional capacities of PBMC populations. Additional research is necessary to determine whether these subtle effects influence the calf's susceptibility to infectious disease.