The frequency of selective deficiency of serum IgA was determined in a population of 64,588 new Finnish blood donors by Ouchterlony's double diffusion with 10 mug/ml as the limit of detection. The incidence was 1:396. Those found IgA-deficient were retested by hemagglutination inhibition and by radioimmunoassay. The calculated incidences of IgA levels below 0.5 and 0.015 mug/ml were 1:500 and 1:800, respectively. Statistically significant compensatory elevation of serum IgG was observed in the IgA-deficient donors. The IgM levels were not changed. Among 9,920 hospital patients, the incidence of IgA deficiency was 1:660. The age structure of the IgA-deficient patients was similar to that of the IgA-deficient healthy blood donors but lower than that of hospital patients in general. No difference was observed between the clinical history of IgA-deficienct blood donors and of the controls.