A monoclonal antibody was developed to the 20 kd major allergen of cow by immunizing mice with crude dander extract. The monoclonal antibody did not exhibit cross-reactivity to cat, dog, and horse dander extracts when studied by ELISA inhibition. The antibody was used in affinity chromatography for the purification of the 20 kd allergen from cow dander extract. Purity of the allergen was estimated to be 88%, and allergenic reactivity was verified by IgE immunoblotting and skin prick tests. After further purification with size-exclusion chromatography, the allergen was almost 100% pure. The isoelectric point of the double-purified allergen was determined to be 4.1. The amino acid composition was characterized by the predominance of acidic amino acids.