Cat allergen-specific serum Ig[immunoglobulin]E antibodies were detected in 27 (71%) of 38 dog dander-sensitive patients. In 4 (11%) of the cases the dual reaction could be explained by the presence of IgE antibodies of cross-reacting serum proteins. In a larger group, the binding of dog dander-specific IgE antibodies could be inhibited by cat epithelium allergens. Two allergenic components of dog dander were separated by chromatography on Sephadex G-75. One of the allergens was closely related to a purified major cat epithelium allergen. The other allergen was apparently dog-specific and showed no cross-reactivity with cat allergens. Both components were present in the dander of dachshund, Airedale terrier, poddle and boxer. The dog dander is the preferable source of dog epithelium allergens provided that the existence of relations to other animals is considered.