An allergen has been isolated from a saline extract of olive tree (Olea europaea) pollen. The protein consists of a single polypeptide chain of 9.2-kDa, as determined by mass spectrometry. It contains neither tryptophan nor tyrosine residues, and displays an acidic isoelectric point. The secondary structure of the protein, estimated from the analysis of the circular-dichroism spectrum in the peptide-bond region, is composed of 52% a-helix, 10% a-strand, 29% Sturn and 9% non-regular conformation. The N-terminal end of the protein is blocked. Amino-acid-sequence data have been obtained from peptides produced by CNBr treatment of the native allergen. A partial sequence of 36 amino acids has thus been elucidated. The protein exhibits sequence similarity with pollen allergens from Brassica species and contains a Caz'-binding motif. The isolated protein displays IgE-binding activity against sera of patients allergic to olivetree pollen. It has been named Ole e 3, according to the recommendations of the IUIS Nomenclature Committee. IgG ELISA inhibition assays with polyclonal antibodies specific for Ole e 3 reveal the presence of proteins similar to Ole e 3 in the pollen from non-related plant species, which may explain allergic cross-reactivity processes.Keywords: allergen ; olive pollen ; Olea europaea ; Ole e 3 ; protein characterization.Type-I allergy is a clinical disorder that affects more than 15 % of the human population in developed countries. Olive pollinosis is one of the most important causes of type-I allergy in Mediterranean countries, where the olive tree is widely cultivated [l]. Ole e 1 has been shown to be a major allergen from olive pollen, since the IgE of more than 70% of olive-allergic patients recognize this protein [2-41. In addition to Ole e 1, other allergenic proteins have been detected in saline extracts of the olive pollen 15-71, some of which exhibit low molecular masses. Preliminary studies on these molecules were performed with pools of sera from patients hypersensitive to olive pollen. Lauzurica et al. 121 showed the existence of an IgE-reacting protein of 8 kDa upon SDS/PAGE of the pollen extract. Proteins of 7, 9, 14, 15 and 16 kDa, present in olive pollen, have been immunostained with individual sera from olive allergic patients [8]. However, isolation or molecular characterization was not performed for these allergens.Allergic cross-reactivities have been described among Oleaceae members. such as olive, ash and privet [9-131 as well as between olive-pollen and grass-pollen proteins [8]. In addition to profilin, which is an important and well defined panallergen involved in cross-reactions [ 141, other proteins contained in pollen from different species that have conserved sequences could be responsible for allergic processes in individuals not previously sensitized to a given biological source.In spite of the knowledge of a high number of allergenic structures, little is known about their B-cell and T-cell epitopes and the mechanism of induction of allergy. The protein allergens fre...