Passive cutaneous anaphylaxis test and Praunitz-Küstner tests were performed in healthy recipient cats with heated and unheated sera of 17 cats suspected of having allergic dermatitis and of 12 healthy control cats. Positive reactions occurred with heated and unheated sera. It was therefore hypothesized that a heat-stabile cytophilic antibody is involved in the pathogenesis of eosinophilic plaques and miliary dermatitis in some cats.