Immune activity during infancy was investigated using blood samples from 30 neonates and 52 healthy infants between 2 and 15 months of age attending for immunization. The purpose of this study was to assess the total immune activity of T cells using soluble interleukin-2 receptor (IL-2R) and interferon-gamma concentrations. These were compared with the proportion of CD4 CD45RO-, IL-2R (CD25)-, and transferrin receptor (CD71)-positive peripheral blood lymphocytes. The median duration of breast-feeding and of introduction of solid feeds was 4.2 and 4.0 months, respectively. Compared to neonates, the mean +/- SE soluble IL-2R concentration peaked at 4 months of age (1670 +/- 94 vs 3060 +/- 252 U/ml; P < 0.0001), as did pooled interferon-gamma levels. The percentage of CD4 CD45RO T cells remained low and the proportion of activated peripheral blood lymphocytes decreased during infancy. We conclude that noncirculatory immune activity is increased during infancy and this is associated with weaning.