Two cDNA clones named P9* and P1* of 794 and 631 bp, respectively, were isolated from a lambda ZAP cDNA expression library using Parietaria judaica (Pj) pollen-specific IgE antibodies from a pool of sera (n = 23) of patients allergic to Pj. Sequence analysis showed open reading frames of 176 and 138 amino acids. Both clones contain a putative signal peptide giving two mature processed proteins named Par j 1.0102 of 14,726 D and Par j 1.0201 of 10,677 D. These proteins represent isoallergenic forms of the major Pj allergen Par j 1.0101 (clone P5) previously reported. The Par j 1.0102 shared 98% amino acid sequence homology with the P5, while the Par j 1.0201 shared 89% homology. Since P1, P5 and P9 clones were expressed in Escherichia coli, and since the three allergenic proteins shared a very high degree of sequence identity and comparable binding to the Pj-specific IgE, we decided to analyze in more detail the immunological properties of only one allergen, the recombinant Par j 1.0101. The allergenic activity determined by the histamine release assay ranged between 9 and 56%, depending on the allergic patient analyzed, while it blocked approximately 40% of all the Pj-specific IgE antibodies, as detected after ELISA and cross-absorption analysis.