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Reagents employed for RAST-inhibition need careful characterization. An allergen reference preparation was produced from carefully selected raw materials. After immunochemical and biochemical characterization, the reference preparation was used to produce allergen reference paper disks. Their quality was controlled by the amount of bound protein and serum titration, which should yield an S-shaped dose-response curve and specific binding of IgE of at least ten times more than background binding. In parallel, the biologic activity of the allergen reference preparations was established in quantitative skin prick tests in at least 20 allergic patients. From these patients, serum samples were collected and pooled after careful characterization. All three necessary reagents for RAST-inhibition (extract, disks, and serum pool) were stabilized by freeze-drying. The reproducibility of the assay procedure itself was checked by documenting the 50% inhibition value of the reference preparation, accepting only values within +/- 2 SD. By employing such reagents we could measure the activity of alum-adsorbed grass pollen extracts with sufficient precision, ie, the coefficient of variation of the mean potency of 12 batches was less than 40%.