Two opposed modes of action have been attributed to immunoglobulin G (IgG) in immediate-type allergy. On the one hand, a small fraction of IgG may have anaphylactic properties. On the other hand, IgG antibodies induced by hyposensitization therapy are considered to act as allergen blocking antibodies. The IgG4 isotype is the major antibody in the immune response induced by hyposensitization therapy and is presumably the only IgG subclass with homocytotropic properties. Immunochemical characteristics of IgG4, such as functional monovalency and inability to activate the complement system, underline the special place of this isotype in the humoral immune response. Detection of IgG4 antibodies in serum has a very limited use in allergy diagnosis. IgG4 antibodies do not appear to have anaphylactic properties, in spite of the homocytotropic activity suggested by some in vitro experiments. However, the assay may be of importance for measuring immune responses in occupational exposure to antigenic substances and in monitoring hyposensitization therapy.