Allergen-specific IgG4 was found in the sera from many normal, nonallergic individuals. Basophil leukocytes from donors with IgG4 antibodies to an allergen, but without IgE antibodies to the same allergen, did not release histamine when challenged with this allergen. In some cases, anti-IgG4 antiserum induced a release of histamine from the leukocytes. However, these cells also release histamine after incubation with normal rabbit serum or normal sheep serum. It is concluded that the IgG4-RAST cannot be used for the detection of IgG short-term sensitizing antibodies.