The aim of the study was to standardize and evaluate technically optimized food allergen extracts for use in skin prick test (SPT). The standardization procedure comprised 36 allergic histories in 32 food allergic patients with 21 healthy, non-atopic individuals serving as controls. The patients had a history of allergic symptoms upon ingestion of either cow's milk (n=3), hen's egg (n=9), wheat (n=4), hazelnut (n=14) or cod (n=6). They also had specific IgE in serum to the food in question and a positive SPT with a fresh preparation of the food. The diagnosis had been confirmed by a double-blind, placebo-controlled food challenge, except for the hazelnut-allergic patients. The controls were subjected to an open food challenge with all the foods to ensure tolerance. The standardization was performed by means of titrated SPT in accordance with the guidelines on biological standardization from the Nordic Council on Medicine. Regression analysis of the skin wheal areas was performed for each patient and the median protein concentration of allergen preparation (median Ch10) eliciting a wheal area of the same size as histamine 10 mg/ml was calculated. The median Ch10 was 0.56 mg/ml for milk, 0.88 mg/ml for egg, 5.4 mg/ml for wheat, 2.1 mg/ml for hazelnut and 0.017 mg/ml for the cod extract. The sensitivity of the median Ch10 estimated from the SPT data was 1 for milk, 0.98 for egg, 1 for wheat, 1 for hazelnut and 0.87 for the cod extract. The allergenic activity of the hazelnut extract was further investigated by leukocyte histamine release (HR) and immunoblotting experiments using sera from 27 hazelnut allergic patients. The clinical sensitivity of the optimized hazelnut extract evaluated by HR was 0.78 compared to 0.30 for a commercially available hazelnut extract (Soluprick). Immunoblotting results showed a stronger IgE binding capacity and additional IgE-binding bands of the optimized hazelnut extract compared with the Soluprick extract.