Twenty-one mild asthmatic patients allergic to cat dander underwent a bronchial provocation test (BPT) with a cat extract. An early allergic response (EAR) was observed in all 21 patients and a late allergic response (LAR) in 8/21 patients. In the EAR, the patients with subsequent LAR had a greater fall in FEV1. Their baseline FEV1, and total dose of inhaled allergen were not significantly different from patients who did not develop a LAR, but their serum specific IgE level was higher. Crossed immunoelectrophoresis (CIE) of the same cat extract showed that it contained eight different proteins. An IgE-CRIE was obtained from all 21 patients, using radiolabelled anti-IgE and autoradiography. Radiolabelled standards allowed a semiquantitative scoring of the radiostaining. The CRIE pattern of the eight patients with LAR showed a higher score of radiostaining and a greater number of proteins bound to IgE. The two major allergen cat albumin and Fel dI bound equally to IgE of patients with and without LAR whereas another protein (antigen No. 7) bound to IgE of 100% of patients with LAR but of only 38% of patients without LAR. These data suggest that the pattern of the IgE response to specific proteins of a cat extract may be related to the occurrence of LAR after BPT with this allergen.