T and B lymphocyte subpopulations were examined in the peripheral blood of children using the E and EAC rosette assays. Children under 18 months of age were found to have a decreased percentage of E-binding (T) lymphocytes and an increased percentage of EAC-binding (B) lymphocytes compared to older children (18 months to 10 years) and adults. The absolute number of E-binding and EAC-binding lymphocytes was increased in children under 18 months of age.