To assess the ability of immunoglobulin production in vivo, we enumerated the immunoglobulin secreting cells in the peripheral blood of patients with an IgA deficiency and of those with hyper-IgA-emia. All seven patients with primary IgA deficiency and two of the three patients with secondary IgA deficiency had low numbers of IgA secreting cells. In all five patients with hyper-IgA the number of IgA secreting cells was increased. Our results suggest that measurement of immunoglobulin secreting cells in PBMCs is useful in the assessment of ability of immunoglobulin production in vivo.