Selective IgA deficiency is the most commonly recognized form of immunodeficiency and is defined as serum IgA undetectable by conventional immunodiffusion methods, with normal or increased serum levels of IgG and IgM. It may occur in association with clinical features, including recurrent sinopulmonary infection, rheumatoid arthitis, SLE or intestinal malabsorbtion, but it is also found in clinically normal subjects. A survey was undertaken in 6191 normal Australian blood donors using an immunodiffusion technique which detected serum IgA down to a level of 10mg/I (normal range for serum IgA levels is 500-3000mg/I). The study found 14 blood donors with selective IgA deficiency (1 in 442 tested).This prevalence rate of 0.23% is the same as that found in blood donors in Sweden but is higher than rates found in France (0.05%), Norway (0.08%), U.S.A. (0.15%), England (0.19%) and Finland (0.20%). Anti-IgA antibodies may develop in subjects with selective IgA deficiency and such antibodies were found in 3 of the 11 blood donors (27%), a rate comparable to that seen in blood donors in U.S.A. and England (18%). By comparison, anti-IgA antibodies occur in approximately 60% of subjects with selective IgA deficiency who present with clinical abnormalities. The importance of anti-IgA antibodies is their potential to cause, in some cases, potentially fatal blood transfusion reactions.