Sera from three hundred five patients with immunoglobulin deficiencies were analyzed for the presence of anti-IgA antibodies by using indirect agglutination and enzyme-linked immunosorbent assay (ELISA). Anti-IgA antibodies were observed in 15 of 68 (22%) patients with hypogammaglobulinemia and 53 of 185 (29%) patients with selective IgA deficiency, both groups having serum IgA less than 0.05 g/liter. The highest frequency, 6 of 10 or 60%, was noted for patients with a combined IgA-IgG2 deficiency. No anti-IgA antibodies were detected in 25 patients with serum IgA between 0.05 and 0.27 g/liter and normal amounts of serum IgM and IgG or in 17 patients with hypogammaglobulinemia who had serum IgA of 0.05-0.7 g/liter. The anti-IgA antibodies were primarily of the IgG class, but IgD and IgM anti-IgA were occasionally found. IgE anti-IgA antibodies could not be detected with the presently used technique. The IgG anti-IgA antibodies were mainly of the IgG1 subclass but occasionally also of the subclasses IgG2, IgG3, and IgG4. Of eight patients with anti-IgA antibodies, seven tolerated Ig prophylaxis with a commercial immunoglobulin preparation low in IgA when given either intramuscularly or intravenously. The titers of anti-IgA in the sera of these patients did not rise in relation to the prophylaxis. Only one of the eight patients had a history of previous anaphylactic reactions to IgA-containing blood products. He tolerated six Ig infusions during 5 months with the IgA-depleted preparation without any adverse effects but showed increasing levels of anti-IgA antibodies and ultimately experienced a near-fatal reaction at the seventh infusion.