The mechanisms of the elevation of serum IgA levels in anaphylactoid purpura were investigated. Serum IgA levels were significantly elevated within 2 weeks (5 to 14 days for all 12 patients) after onset in patients with anaphylactoid purpura. Serum IgM and IgG were not elevated. Although the percentages of surface IgA-bearing cells were not increased in the patients, the numbers of IgA-secreting cells within 2 weeks after onset in the patients with anaphylactoid purpura were significantly higher than those of controls. In northern blot analysis on lymphocytes, the secreted alpha (alpha s)-chain gene was well expressed compared with the membrane-bound alpha (alpha m)-chain gene, within 2 weeks after the onset of anaphylactoid purpura. Therefore, stimulation by a certain agent or a certain immune response may accelerate expression of the alpha s-chain gene in anaphylactoid purpura.