The red cell-linked antigen-antiglobulin reaction (RCLAAR) with stearoyldextran-coated erythrocytes was used to characterize the immunoglobulin (Ig) classes and IgG subclasses of dextran reactive antibodies (DRA) in 27 dextran reactors (DR) and 96 on reactors (DNR). High titres of dextran reactive IgG were regularly found in sera of patients with severe dextran-induced anaphylactoid/anaphylactic reactions (DIAR) prior to the infusion. In four lethal cases IgG antibodies were found to be in the highest titre range of 16,384 to 32,768. In addition, high IgA and IgM titres were also in severe DIAR. DNR had much lower titres of dextran reactive antibodies of IgG, IgM and IgA classes and IgD antibody was absent in both groups. Dextran reactive IgE antibodies were not demonstrable in DR. Dextran reactive IgG2, IgG3, IgG4 and IgG1 (indirect measurement) were demonstrated in both DR and FNR. Dextran infusion caused variable neutralization in all Ig classes and IgG subclasses, but the contribution of IgG2 was considered most important because of its high titres and most pronounced neutralization in severe DIAR. It is concluded that DRA mainly of the IgG class, play a critical pathogenic role in the induction of severe DIAR, which accordingly is classified as immune complex (Type III) anaphylaxis. The method of RCLAAR allows to delineate a risk group of about 2% of potential reactors.