Total antibody binding of ragweed antigen E (AgE) and IgE antibody of AgE were quantitatively measured in serum from eleven patients given immunotherapy with purified AgE. From these data the contribution of antibody other than IgE, presumed to be mostly IgG, to total AgE binding could be determined. Binding of AgE by antibody other than IgE antibody was considered to be due to blocking antibody. As immunotherapy progressed, IgG antibody binding of AgE and the IgG/IgE binding ratio were serially determined. IgG antibody binding of AgE increased from a pretreatment mean of 238 ng AgE bound/ml to 3142 ng AgE bound/ml just prior to the first ragweed season and reached a peak of 4286 ng AgE-bound/ml. Although blocking antibody thus increased progressively with treatment it was not correlated significantly with symptom scores. The IgG/IgE binding ratio increased from a pretreatment mean of 19-290 just prior to the first ragweed season and reached a peak of 1167. This ratio was related significantly to symptom scores reported by patients during the ragweed season subsequent to immunotherapy.