Early in life, the cells involved in immune selection differentiate into CD4+ and CD8+ cells. Currently there is insufficient information on the cell maturation and activation of these peripheral blood mononuclear cell (PBMC) subsets in normal children. The critical need for this information has been brought about by the pediatric HIV/AIDS epidemic, which requires the measurement of peripheral blood CD4+ T cells and other cells types for interpretation of HIV disease progression. The immunopathogenesis of pediatric HIV infection differs from that in adults but is not well understood. In order to better understand HIV disease progression in HIV-infected children, these PBMC subsets must be studied in normal children so that control values can be established.