Labeled antigen-binding tests were used to determine quantitatively the contribution of IgG4 antibodies to the total IgG antibody response in humans. In agreement with literature, we found no IgG4-restricted antibody responses with tetanus toxoid or streptococcal carbohydrate. In the serum of individuals immunized for several years with phospholipase (PLA) from honey bee venom, grass pollen allergen, or house dust mite allergen, we often found that more than 50% of the total antigen-binding capacity was due to IgG4 antibodies. In the case of beekeepers, it could clearly be shown that during prolonged immunization a shift in the IgG4:IgG1 antibody ratio occurs that finally results in an IgG4-dominated antibody response. Evidence is provided that antigen-binding assays may even underestimate the contribution of IgG4 antibodies, because in contrast to IgG1 antibodies, IgG4 antibodies act as monovalent antibodies in being unable to cross-link immunosorbent-bound antigen and radiolabeled antigen.