Antibodies to IgA were detected in two males with hypogammaglobulinaemia. The antibodies were demonstrated to belong to the IgG class. In both patients low serum levels of IgG and IgM but no IgA were found. On two occasions one of the patients reacted with severe side reactions within a few seconds after an intramuscularly given injection of gammaglobulin. The other patient had received monthly intramuscular injections of gammaglobulin over a period of 2 years when the anti-IgA antibodies first appeared and has since then been regularly treated for 1 1/2 years without any signs of adverse reactions. In five additional patients with hypogammaglobulinaemia and side reactions after i.m. injections of gammaglobulin no antibodies to IgA, IgG or Gm-determinants were detected.