Immunological and genetic studies were performed in nine members from three generations of the family of a patient with common variable immunodeficiency (CVI). Two additional symptomatic members (mother and grandmother) had CVI. Among other six asymptomatic members, two had CVI and one had selective IgA deficiency. The proportions of monoclonal antibody defined total T cells (Leu 1+), helper phenotype (Leu 3+) suppressor phenotype (Leu 2+) T cells, natural killer cells (Leu 7+) and surface Ig+ B cells and proliferative response to phytohaemagglutinin (PHA), concanavalin A (Con A), pokeweed mitogen (PWM) and in mixed lymphocyte reaction (MLR) were comparable to controls. Addition of purified interleukin-2 (IL-2) resulted in augmentation of PHA-induced proliferation of T lymphocytes similar to that seen in the controls, however with IL-2 freshly isolated T cells in the absence of PHA demonstrated markedly increased proliferative response, suggesting the presence of in vivo activated T cells. Study of HLA phenotype did not reveal any linkage. This study demonstrates the genetic nature, possibly autosomal dominant inheritance, of common variable immunodeficiency; however the immunodeficiency is not linked to any specific HLA antigen.