The aqueous extract of inflorescences of Parietaria judaica contains an allergen homologous to the major pollen allergen Par·I (14 kD), as shown by radio-allergosorbent test (RAST) inhibition and immunoblot analysis. Poly(A)+ RNA was obtained from inflorescences and was shown to be able to code in vitro for a protein homologous to Par·I with respect to sodium dodecylsulphate polyacrylamide gel electrophoretic mobility and to antigenic specificity as defined by the binding, in affinity chromatography, to solid-phase IgG of rabbit anti- Par·I antisera, and in RAST inhibition, to IgE antibodies of human reaginic serum pool.