An allergenically active basic protein, antigen Ra5, was isolated from DEAE-fraction C of the aqueous extract of short ragweed pollen by gel filtration and cation exchange chromatography. The antigen was homogeneous by a number of physicochemical and immunological criteria, and showed no cross-reaction with ragweed pollen antigens E, Ra3 and BPA-R. It was devoid of carbohydrate, had a molecular weight of 5,130 ± 50 in the analytical ultracentrifuge and the following composition: lys4 arg2 asp2 ser4 glu4 pro3 gly4 ala3 (cys/2)6 val3–4 ileu leu1–2 tyr3 try2.