Background: Antiallergen mAbs that do not recognize clinically important isoforms have been described, raising the question of the selection of mAbs for quantifying major allergens in order to standardize allergenic extracts. This question is even more critical if mAbs can discriminate between different forms of allergen molecules with the same amino acid sequence. Objective: We sought to demonstrate that an anti-Fel d 1 mAb was able to discriminate between two forms of the major cat allergen independently of its amino acid sequence and to determine the relative importance and stability of both forms in various cat extracts. Methods: Anti-Fel d 1 mAbs were raised in mice and characterized. By using two of these mAbs, a two-site ELISA was developed to quantify Fel d 1 in mass units. Results: One of the anti-Fel d 1 mAbs developed was shown to specifically recognize a particular form of Fel d 1. A two-site ELISA with this mAb to capture Fel d 1 was able to quantify the allergen specifically in this form. It was then shown that (1) the quantitative importance of this form of Fel d 1 could vary from one cat extract to another, (2) Fel d 1 was converted into this form under certain conditions, and (3) both converted and unconverted forms of Fel d 1 may bear IgE epitopes that are specific. Conclusion: Although the present study emphasizes the issue of selecting mAbs that are not too specific to standardize allergenic extracts, it also demonstrates that very specific mAbs can be of interest, especially to verify the stability of allergens in extracts, since this stability might have clinical implications. (J Allergy Clin Immunol 2000;106:669-76.)