As basophils are the major effector cells of allergic reactions, confirmation of the allergic etiology of chronic obstructive pulmonary disease (COPD) was sought by the demonstration of a specific in vitro response of equine basophilic blood cells to some potential allergens (Aspergillus, Cladosporidium, Mucor, Penicillium, extracts of dust particles of hay and straw). The allergen induced degranulation of basophils and the histamine and protease release from basophils during incubation with the allergens were tested. By evaluating the results obtained from 14 COPD horses and eight controls it could be shown that the sensitivity of the basophils of affected horses was increased, particularly against the allergen extract of Mucor mucedo and Mucor spinosus. Further a greater percentage of COPD horses reacted positively with the Mucor allergen extract. The mitogenic stimulation of lymphocytes by PHA and by the allergen extracts used gave comparable results in affected and control horses. Thus the in vitro stimulation of basophils may be an easily to perform testing device for the identification of potential allergens involved in the pathogenesis of equine COPD.