A low mol. wt allergen (Pj-2) and a hapten (Pj-H3) were purified from Parietaria judaica pollen by means of long-term aqueous extraction, dialysis and gel filtrations. The yield of the Pj-2 allergen was 0.94% (w/v) of the total protein present in the aqueous extract of the pollen, while its allergenic activity was about 60% of the total dialyzable activity, as verified by skin prick tests, ELISA- and RAST-inhibition experiments. The homogeneity of this allergen was demonstrated by one single sharp peak on HPLC, one single band on PAGE-SDS and by one single arc on IEF. Its mol. wt, estimated by HPLC and amino acid composition, was 10,400. The amino acid analysis showed 73 amino acid residues, and lysine was predominant, with 20 residues. The hapten Pj-H3 was 0.2% (w/v) of the total protein found in the pollen aqueous extract. It was inactive in skin prick tests even at a protein concn of 2 mg/ml, while it was capable of inhibiting by 60% in ELISA- and RAST-inhibition experiments, suggesting an immunochemical relationship with both IgE and allergens specific to P. judaica. The homogeneity was demonstrated by one single sharp peak on HPLC and one single band on PAGE-SDS. The amino acid analysis showed 10 amino acid residues, with no specific traits, and the mol. wt determined by gel filtration and amino acid composition was 1000. An immunochemical relation between the allergen and the hapten was also suggested by the results of an ELISA-inhibition test, and by the ability of the hapten to partially inhibit the precipitin line between rabbit antibodies to whole P. judaica pollen extract and the Pj-2 allergen. The allergen and the hapten described above, purified at homogeneity and in an antigenically active state, both provide adequate material for further structural and immunological characterizations.