The allergenic components of cat fur extract were localized in the molecular weight range 65,000-40,000 and had a pI of 3.9-5, except one with a more acid pI of 3.5. The fraction with a mean molecular weight of 65,000 contained proteins of pI 4.5-5 and mainly cat serum albumin. The fraction with a mean molecular weight of 40,000 showed two peaks of activity. One of them was associated with a protein of pI 3.5, and immunodiffusion analysis showed the presence of 'Cat Allergen One'. A comparative study on cat saliva showed that the allergenic activity was restricted to proteins of the same molecular weight range. 'Cat Allergen One' was present in the fraction of molecular weight 40,000.