For determination of allergen-specific IgE in cell culture supernatants and other highly diluted IgE preparations a radioallergosorbent test (RAST) based on high adsorption polystyrene test tubes has been developed ("Maxisorp RAST"). Cladosporium herbarum extract was used as a model allergen but timothy grass pollen, house dust mite and dog dander showed similar results. The test showed specificities of both allergen and immunoglobulin isotype and significant correlations (r = 0.67-0.88) with established RAST procedures were found. Based on immunosorbent-purified allergen-specific IgE the estimated sensitivity was within the order of 150-300 pg allergen-specific IgE per ml. The within-assay variation was 4-9% and the inter-assay-variation 17-29%. The Maxisorp RAST is useful as an inhibition assay for quantitating allergenic activity down to 0.1 biological units/ml of allergen extracts.