Forty patients with a positive case history and specific serum IgE to cat allergens were evaluated using titrated skin prick test (SPT), histamine release studies (HR) and immunoblotting studies. To determine the relevance of the major allergen, a monospecific antibody to Fel d I was preincubated in different concentrations with the cat allergen dose required for maximum response in the HR assay (10 SQ/ml). Dose-dependent inhibition of histamine release (n = 30, x = 66% +/- 5% IC30 = 0.36 micrograms/ml) was found in all patients sensitized to cat allergens as shown by antigen-induced HR. The greater the HR response to the cat allergen extract the higher were the antibody concentrations necessary for inhibition (p < 0.05). Immunoblot experiments (n = 22) using SDS-PAGE demonstrated five different locations of IgE binding in 15 sera: 18, 35, 46, 54 and 66 kD. IgE binding at 18 kD was found in 14 patients. Anti-Fel d I possessed two strong bands at about 18 and 35 kD which are representative of the major allergen Fel d I. Our findings emphasize the major role of Fel d I in patients sensitized to cat allergens.