The antigenic characteristics of three preparations of ragweed (RW) antigen E were studied in rabbits. The preparations consisted of the monomeric form, (m.w. 37,800); a lower molecular weight polymer (m.w. 0.2 to 4 million) and a higher molecular weight polymer (m.w. 4 to 20 million). Antibody responses of two Ig classes occurred. A circulating antibody response detectable by an ammonium sulfate-binding technique was consistent with an IgG response. A homocytotropic antibody response, probably rabbit IgE, occurred in a majority of animals. The polymerized RW preparations appeared as antigenic as the monomer in induction of the IgG response. The low molecular weight polymer resulted in a lower and later reaginic antibody response than either the monomer or high molecular weight polymeric preparation.