To assess the relative potency of the major peanut allergens, Ara h 1 and Ara h 2, we examined the relative ability of purified proteins to bind IgE on immunoblots, to cross-link allergen specific IgE in an in vitro assay of degranulation based on RBL SX-38 cells, and to bind IgE in the ImmunoCap assay. Sera from 12 highly sensitive, peanut allergic patients were studied in all assays. IgE immunoblots with crude peanut extracts showed binding of IgE to multiple bands including the 63 kDa and 17-19 kDa bands that contain Ara h 1 and Ara h 2, respectively. In the functional assay, Ara h 2 was more potent than Ara h 1 in 11 of 12 sera tested with a median potency that was 52.5-fold more than Ara h 1 (P < 0.005). Contrary to findings with the functional assay, IgE immunoblots with purified Ara h 1 and Ara h 2 showed substantially lighter binding of IgE to Ara h 2 compared with Ara h 1 (P = 0.02). The ImmunoCap assay gave intermediate results with slightly more IgE binding to Ara h 2 than to Ara h 1 (P = 0.005). In conclusion, Ara h 2 is a very potent allergen and is much more potent than Ara h 1 for most sera using an in vitro assay of IgE cross-linking and cell activation. This finding is different from what was predicted based on immunoblots or with the ImmunoCap assay.