Background: The peach lipid transfer protein Pru p 3 has been identified as a major allergen from this fruit. Homologous cross-reactive allergens have been found in several plant foods and pollens. Recombinant Pru p 3 has been recently produced in the yeast Pichia pastoris . Objective: We sought to evaluate the potential role of recombinant Pru p 3 as a novel tool for the diagnosis of fruit allergy. Methods: Circular dichroism analysis was used to compare the protein folding of natural Pru p 3 and recombinant Pru p 3. IgE binding by both molecular forms was quantified by means of ELISA and ELISA inhibition assays, and their biologic activity was estimated by using basophil activation, histamine release, and sulphidoleukotriene production tests. Individual sera or blood samples from patients with peach allergy (up to 17) were used in the assays. Results: A nearly identical circular dichroism spectra was shown by using natural Pru p 3 and recombinant Pru p 3, indicating that both protein forms are similarly folded. No difference was detected in the IgE-binding capacity of the 2 mo-lecular versions. Basophil activation and induction of sulphidoleukotriene production were positive in 9 of 10 patients, and histamine release was induced in at least half of the patients, with similar effects of the natural and recombinant forms in the 3 assays. Conclusion: Recombinant Pru p 3 shows a strong immunologic activity equivalent to that of its natural counterpart, and therefore it can be a useful tool for diagnosis (and future immunotherapy) of fruit allergy. (J Allergy Clin Immunol 2003;111:628-33.)