A major allergen, the Parj I, was purified to homogeneity from Parietaria judaica pollen by means of ultrafiltration dialysis, preparative polyacrylamide gel chromatography and affinity chromatography through a column of Sepharose-monoclonal antibody specific for Parj I. The homogeneity of the Parj I was assessed by one single arc of immunoprecipitation both in cross immunoelectrophoresis (CIE) and crossed radioimmunoelectrophoresis, by one single band of radiostaining after a sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transfer to nitrocellulose and by one single peak after a size exclusion chromatography on high-performance liquid chromatography (HPLC). The homogeneity was further supported by crossed Laurell immunoelectrophoretic analysis, in that only one arc of precipitation was magnified in CIE after addition of the purified allergen. The purified Parj I allergen was capable of interacting in vitro with 70% of the human IgE specific for a crude P. judaica extract, as determined by radioallergosorbent test inhibition. The purified Parj I was capable of inducing positive reactions in vivo in skin prick tests, and of inducing release of histamine from blood containing basophils as determined by a histamine release assay. The amino acid analysis of the Parj I showed 118 amino acid residues per monomer analyzed and, among other residues, three methionine residues were detected. The molecular weight of the Parj I estimated by HPLC and amino acid composition was 26 kilodaltons.