Monoclonal antibodies and flow cytometry are now used routinely in the diagnosis of many malignant diseases and primary and secondary immunodeficiency states. Technical advances have improved the identification of blood lymphocyte subsets and reliable normal values are now obtainable. Such values have been reported for adults but not for children. We report both absolute and percentage normal values for lymphocytes and their subsets in infants and children of different ages. Our findings show that the absolute and percentage values for most lymphocyte markers differ substantially not only between children and adults, but also between children from different age groups. In infants, erythroid cell contamination of Ficoll gradient-density isolated mononuclear cells must be removed to obtain reliable flow cytometry values.