Investigations of cross reactions between a cat hair and skin scraping extract (CHSS) and a cat serum (CS) were done by several biochemical and immunochemical methods. Furthermore, we looked for common allergenic molecules in a cat 1 reference preparation and in a CS. In crossed (radio) immunoelectrophoresis (CIE/CRIE) of CHSS vs. anti CS-serum one to eight allergen bands could be detected depending on the human serum employed. By constructing allergograms from the CRIE patterns of 17 patients' sera one out of eight allergens could be defined as a major allergen. CRIE of the cat 1 reference vs. anti CS-serum, using the sera of two cat allergic patients, displayed four to nine allergen bands, indicating that the cat 1 reference is not a pure preparation but rather an enriched fraction.