We describe a novel procedure for standardization of allergen extracts using radioiodinated crude extracts. Radiolabeled extracts were reacted with immunoglogulin E (IgE) antibody, and the resulting complexes precipitated by anti-IgE. Experiments with ragweed pollen, grass pollen, honeybee venom, and Alternaria extracts revealed that 1% to 14% of the radioactivity was precipitated, these percentages ranged from 2 to 20 times blank values. Binding of labeled ragweed extracts to IgE antibody could be inhibited by unlabeled extracts and linear inhibition responses could be derived. The potencies of 11 short ragweed extracts were ranked by their inhibitory capacities in such an IgE radioimmunoprecipitation (RIP) test and compared to potencies determined by radioallergosorbent test (RAST) inhibition; these were related, rs = +0.97 (p less than 0.001). Labeled extracts were also reacted with IgG antibodies, and the resulting complexes were precipitated by anti-IgG. From 1% to 34% of the radioactivity associated with the various extracts was precipitated by anti-IgG, and these percentages were from 2 to 20 times blank values. The reaction of IgG antibody with radiolabeled pollen extracts could be inhibited by unlabeled extract, and in the case of short ragweed the measurements of potencies using IgG and IgE antibodies were correlated, rs = +0.96 (p less than 0.001), suggesting that similar determinants were recognized by both IgG and IgE classes. The RIP procedure provides an alternative to the RAST for the standardization of allergy extracts and eliminates the need for affinity chromatography-purified antibody to IgE.