Twenty-four asthmatics allergic to cat and/or dog dander were included in a study to examine the efficacy and safety of immunotherapy (IT) with partially purified, standardized extracts of cat or dog dander. In the first placebo controlled, double-blind part of the study, 10 patients were treated with extracts of both eat and dog, 12 with cat extracts and 2 with dog extracts. Fifteen patients received active IT and 9 placebo injections. Patients treated with both extracts received active extracts only, or placebo only. Bronchial allergen challenge after 5 months demonstrated a significant fall in sensitivity to eat (P = 0.04) in patients treated with cat extracts. No significant changes were found in sensitivity to dog after treatment with dog dander extract or in the placebo groups. During this period, bronchial sensitivity to histamine did not change significantly in any of the groups. To examine the effect of more prolonged IT, 19 patients allergic to cat (17) and/or dog (9) were treated for 12 months. Bronchial sensitivity to cat decreased further (P = 0.003), while no significant change was found in dog extract-treated patients. In cat extract-treated patients a significant decrease in bronchial histamine sensitivity developed (P = 0.02). Systemic side effects were few, but in some eases, local side effects were a dose-limiting factor. This study demonstrated that IT with cat extract may benefit cat-allergic asthmatics, whereas no influence of IT with dog extract was detected in dog-sensitive asthmatics.