Ragweed antigen E polymerized by treatment with glutaraldehyde offers a potentially improved method for immunotherapy of human IgE-mediated pollinosis because studies have demonstrated that the polymer retains antigenic reactivity but has decreased ability to release histamine when these functions are compared on a weight basis. The potential clinical usefulness of the polymer is dependent both on stability of the polymer and on retention of antigenic reactivity. The method used for the analysis of the stability of the polymer under conditions of storage was the comparison of gel filtration fractionation profiles. No depolymerization was demonstrated during storage conditions for 2 mo, and only minimal evidence of additional polymerization (less than 8%) or depolymerization (less than 7%) was observed after 6 mo of storage. There was no difference in the per cent of polymerized antigen E precipitated by rabbit antiserum after storage as compared with control samples.