Glutaraldehyde treatment of partially purified ragweed AgE was done to provide cross-linked proteins of this material. The ragweed preparation contained a high concentration of antigen E. The glutaraldehyde treatment produced aggregates of ragweed antigen E that were fractionated through Sepharose 6B, 4B and Sephadex G-200 to prepare polymers of two molecular weight ranges, the first approximately 0.2 to 4 million and the second 4 to 20 million. These materials reacted completely with antibody against ragweed antigen E but required higher concentrations to neutralize antibody as the molecular weight of polymerized ragweed increased. The polymerized ragweed did not show antigenic deletion nor were new antigenic determinants definitively shown. The polymerized ragweed was at least as antigenic in animal studies as ragweed antigen E and the polymerized ragweed antigen with a molecular weight range of approximately 0.2 to 4 million showed evidence of possibly greater antigenicity than the monomeric ragweed antigen E.