A fractionation method has been developed for the preparation of a basic protein antigen (BPA-R) from ragweed pollen. This antigen possesses potent allergenic activity. It is homogeneous by gel filtration on Sephadex G-100 and carboxymethyl-Sephadex chromatography. By the former technique the molecular weight of the BPA-R was estimated to be 28 000. Judged by its physicochemical and immunologic properties the BPA-R appears to be distinct from several other highly purified antigens currently isolated from ragweed pollen.The BPA-R and AgE (a highly purified acidic protein antigen) when tested together in Ouchterlony analysis displayed reactions of typical partial immunological identity with rabbit anti-ragweed sera which were produced by hyper immunization utilizing complete Freund's adjuvant. In contrast rabbit antisera, produced on a shorter schedule with BPA-R (using incomplete Freund's adjuvant), failed to react by Ouchterlony test with the AgE but reacted strongly with the BPA-R. Further immunological as well as chemical studies will be necessary to elucidate the antigenic relationship between these moieties.