An assay for estimation of quantity and quality of antigen-specific IgG against two purified antigens from cow hair and dander is described. The substance concentration of antigen-specific IgG was more than 10 times higher in serum from immunized patients than in serum from non-immunized patients. Almost the same KA values of 5 x 10(9) l/mol were calculated for IgG in sera from immunized and non-immunized patients with allergic asthma to cow hair and dander.