The peripheral blood leukocytes of 6 children with clinical data suggestive of primary cellular immunodeficiencies were studied in an attempt to define the cellular basis of these disorders. The phenotype and function of T and B cells were investigated. According to the clinical and laboratory features, the patients were classified as one case of severe combined immunodeficiency (SCID), two of ataxia-telangiectasia (AT), one of Wiskott-Aldrich syndrome (WAS), one of DiGeorge syndrome (DSG), and one of cellular immunodeficiency (CID). The laboratory investigations together with the clinical manifestations permitted a diagnosis of primary immunodeficiency diseases.