A crude (saline soluble) extract of cat skins capable of eliciting a strong positive prick skin test in cat sensitive individuals was fractionated on Sephadex G200. Active fractions were pooled and successively fractionated on isoelectric focusing gradients of pH 3–5 and pH 4–5. Allergenic activity was localized in two peaks with mean isoelectric points of 4.1 and 4.35 respectively. On immunoelectrophoresis the allergen with pI = 4.35 was associated with a protein which was subsequently found to be immunologically indistinguishable from serum albumin and to have a molecular weight of 69,000 daltons. The allergen with pI = 4.1 migrated in the α2-region on immunoelectrophoresis and had a mean molecular weight of 55,000 daltons. This allergen was isolated and analysed for amino acid and carbohydrate content.

A combined extract of both allergens coupled to microcrystalline cellulose and used in a RAST procedure readily distinguished between two groups of individuals classified as skin test positive and skin test negative to cat allergen.