The reaginic activity in sera from ragweed-sensitive patients was inactivated after the precipitation of serum proteins with the rabbit antihuman globulin serum which did not contain any of the antibodies specific for γG-, γA-, γM-globulins and for light chains of immunoglobulins. The rabbit antiserum gave a γ₁-globulin precipitin band with the reagin-containing fraction which contained γA but none of the γG-, γM- or γD-globulins. The specificity of the precipitin band was different from the γA-anti-γA band. By radioimmunoelectrophoresis and by radioimmunodiffusion, the antibody against ragweed pollen-extract (antigen E) was detected in the γ₁-globulin, indicating the presence of a unique γ₁-immunoglobulin. The protein was tentatively designated γE-globulin. Precipitation of γE-globulin anti-ragweed antibody in the reagincontaining fraction was accompanied by the loss of reaginic activity, whereas the precipitation of the γA-globulin antibody in the fraction did not result in any decrease of reaginic activity. The results highly suggest that the reaginic activity is associated with γE-globulin.