A series of monoclonal antibodies to T cell surface antigens were used to characterize peripheral lymphoid populations from patients with a variety of immunodeficiency diseases. Several disorders of T cell differentiation were observed to occur in severe combined immunodeficiency. One subtype of severe combined immunodeficiency was associated with failure to develop lymphocytes that express any thymus specific antigens, another with failure to differentiate beyond the early prothymocyte-thymocyte (T9+, T10+) stage, while a third subtype was associated with failure to differentiate beyond a late thymocyte (T3+, T4+, T5+/T8+, T10+) stage. In contrast, patients with thymic aplasia (DiGeorge syndrome) had a diminished but detectable population of mature T cells. Imbalances in immunoregulatory T cells with a relative excess of suppressor cells were found in 9 of 17 patients with spontaneously occurring acquired agammaglobulinemia. In one of the latter individuals, there was an activated suppressor T cell population expressing Ia antigens (T+/T8+, Ia+). Another had no inducer T4+ cells. Patients with X-linked agammaglobulinemia frequently had an abnormal ratio of inducer to suppressor cells as well as an absence of circulating surface immunoglobulin-bearing cells. No such abnormalities were noted in normals or individuals with selective immunoglobulin (Ig)A deficiency. Taken together, these findings support the notion that several immunodeficiency states may occur as a consequence of defective T cell maturation or imbalances in immunoregulatory T lymphocyte subpopulations.