The prevalence of antibodies to immunoglobulin A (IgA) was studied in eighty-three subjects with selective IgA deficiency (SIgAD), thirty-three normal individuals, thirty-two children with juvenile rheumatoid arthritis (JRA) and normal or elevated IgA, seventeen children with systemic lupus erythematosus (SLE) and normal or elevated IgA, and thirteen patients with hypogammaglobulinaemia. Anti-IgA antibodies were detected by haemagglutination of human erythrocytes coated with one of two IgA myelomas by the chromic chloride technique. Antibodies to IgA were not found in significant titre in individuals with normal or elevated IgA or in those with hypogammaglobulinaemia. In IgA deficient patients, elevated titres were found in 25--30% of those who were healthy or who had non-rheumatic diseases, 50% of those with rheumatoid arthritis, 77% of those with JRA and 100% of those with SLE. Patients with rheumatic diseases and SIgAD may therefore be particularly at risk of developing anti-IgA-mediated reactions to blood products.