Cat epithelia extracts were found to contain several electrophoretically separable allergens. Some of these were also detected in cat serum. In addition to serum proteins the epithelia extracts contain allergens with higher electrophoretic mobility than albumin, which was found to be the main allergen of cat serum. The molecular weight of the main cat epithelia allergen was about 32,000. 56 sera from allergic patients were studied by means of the radioallergosorbent test using cat epithelia extracts, a purified epithelia allergen fraction and cat serum as test allergens. About 7% of the patient sera were found to contain IgE specific to cat serum proteins. IgE specific to the epithelia allergen fraction, which was free from serum proteins, was found in 66% of the patient sera, while the rest, about 27%, contained IgE reacting with both kinds of allergens.