In evaluating the therapeutic efficacy of gammaglobulin preparations in patients with agammaglobulinemia, opsonic activity of patients' sera in terms of oxygen consumption capacity of normal leukocytes is advocated as a useful index. Although the serum IgG level is a commonly used indicator, its determination alone was found out not always to reflect antibody levels in such sera. The method described here is simple and gives quantitative results and deserves a wider trial in controlling gammaglobulin replacement therapy.