Fraction A contains all of the major antigens and allergenic properties of whole ragweed extract, but lacks the yellow pigment and some low molecular weight substances. In a double blind study, the clinical and immunoiogical responses were observed following administration of aqueous and alum precipitated extracts of fraction A to 21 previously nonimmunized ragweed sensitive patients. Both preparations of fraction A were clinically effective. The reactions which occurred after the alum and aqueous extract injections were similar in incidence and character. The immunoiogical response following both extracts was also quite similar. In the majority of patients, serum total IgE levels rose, both as the result of immunotherapy and seasonal exposure. Ragweed radioallergosorbent (RAST) antibody levels, a measure of reagin, dropped in two thirds of the patients following therapy and remained unchanged in all patients during inhalant pollen exposure. Hemagglutinating antibody titers generally rose. Aqueous and alum precipitated fraction A extracts were clinically and immunologically effective. However, further studies are needed to establish any advantage of fraction A over present whole ragweed extract preparations.