Soon after the initial isolation of allergen Ra5 (1) from the pollen of short ragweed (Ambrosia elatior), human IgE-mediated sensitivity to this 5000 m.w. protein was shown to be significantly associated with the HL-A7 creg (cross-reacting group) of histocompatibility antigens (2, 3, 5). With determination of its amino acid sequence (6), Ra5 emerged as a potentially useful model for structure-function correlations in atopic hypersensitivity (4, 6, 7). However, extensive studies on the allergen have been hampered by the relatively low yields (approx. 10 mg/kg) obtained with existing isolation procedures (8) which have among other things utilized ether-extracted pollen as starting material. Following the finding by Marsh (3) that the aqueous extract formed from non-defatted pollen contained 330 to 350 mg antigen/kg, as determined by double antibody radioimmunoassay with anti-Ra5 serum, we restructured our method of isolation based on omission of the ether extraction step. As a result, we can report appreciable increase in yield of Ra5 in homogeneous form.