T-cell phenotypic analysis with anti-T monoclonal antibodies (MoAb) was performed on 37 children with immunologic disorders. Abnormalities of T-cell differentiation and/or of T-cell subset distribution were observed in many patients. In particular two infants with severe combined immunodeficiency showed immunologically incompetent common thymocytes (OKT6+) in the circulation, in one case a proportion of OKT6+ cells was OKT4-, OKT8-. A boy with a selective T-cell defect synthetized normal levels of Ig classes, despite the marked reduction of helper/inducer T-cells (OKT4+). Irregularities of T-cell subsets were also noted in children with Wiskott-Aldrich syndrome and in some patients with selective IgA defect or hypogammaglobulinaemia. In one of these, in whom the agammaglobulinaemia was caused by EBV infection, a persistently reversed OKT4+/OKT8+ ratio together with an excessive suppressor T-cell function were found more than 10 years after the onset of the disease. Such a case supports the hypothesis that a viral infection may cause, in a predisposed host, both the agammaglobulinaemia and an abnormality of the regulatory T-cell subpopulations. Such abnormalities, together with those found in the other children studied, underline the importance of MoAb against different T-cell antigens for a better characterization of primary immunodeficiencies.