Mice of 6 strains were immunized with a highly purified Fel dI allergen adsorbed to alum. Their ability to display a significant IgE response was detected via passive cutaneous anaphylaxis (PCA) tests, performed in rats. Linkage of the responsiveness to the H2 genotype is not totally obvious. IgE response can be delayed, particularly in the SJL strain (H2-s histocompatibility allele), and lacking in the C57 B1/6 strain (H2-b). Mice with H2-k, H2-d alleles and the B6D2F1 hybrid H2-b/d are good responders, leading to the hypothesis that the IgE response to Fel dI may be related to the H2-d allele. For all good responders, individual variations are rather important. All our observations show that mice perfectly mimic human hypersensitization to the cat major allergen Fel dI, at least where the IgE response is concerned.