Abstract Regulator cell function and terminal differentiation of B lymphocytes in six patients with selective IgA deficiency were evaluated. Suppression of IgA synthesis was determined by in vitro pokeweed mitogen-stimulated cell cultures in which IgA-deficient and control lymphocytes were cocultured for 7 days. IgA synthesis was measured by double antibody radioimmunoassay in culture supernatants. Ficoll-Hypaque-isolated peripheral blood lymphocytes from all six patients with selective IgA deficiency were found to synthesize significantly smaller amounts of IgA following pokeweed stimulation than did the normal controls. Cocultures of IgA and control lymphocytes revealed a significant inhibition of expected IgA synthesis in three of the six patients. Concomitant control lymphocyte cocultures showed little or no inhibition. These results suggest the presence of suppressor cells for IgA synthesis in a subpopulation of patients with selective IgA deficiency.