A panel of previously characterized monoclonal antibodies: OKT3, OKT4, OKT8, OKT10, OKT11, OKIa1, OKM2; 3A1, 4F2, UCTH1 and 5/9 were used to evaluate peripheral blood mononuclear cells in patients with severe primary immunodeficiencies: three patients with severe combined immunodeficiency, five with X-linked agammaglobulinemia, 20 with common variable hypogammaglobulinemia, 11 with IgA defect, and one with an unclassified form of T cell defect and hypogammaglobulinemia. Surface markers for T and B cells and in some cases functional assays, were also performed. Our results indicate a heterogeneous pattern in patients with severe combined immunodeficiency: one had peripheral blood mononuclear cells negative with all the monoclonal antibodies used; one had an increase in OKM2+ cells, whereas OKT3+ cells were absent; one had defect and imbalance of immunoregulatory T cell subpopulations. Major imbalances of T cell subsets were not detected in patients with X-linked agamma and IgA defect, whereas in some patients with common variable hypogammaglobulinemia an inversion of the physiological ratio between OKT4+ and OKT8+ cells was consistently detected. In an unclassified case of primary immunodeficiency, almost all peripheral blood mononuclear cells formed rosettes with sheep erythrocytes, but lacked antigens detected by monoclonal antibodies. Based on these observations, possible sites of defects in the T cell differentiation are discussed. We believe that monoclonal antibodies are useful for diagnosis, classification, and monitoring of therapy of primary immunodeficiencies.