A role for platelets in allergic airways disease has been postulated and changes in the responsiveness of circulating platelets have been demonstrated following antigen challenge of asthmatic human subjects. In this study agonist-induced aggregation of equine platelets in vitro has been compared before and after exposure of horses to a controlled hay and stray challenge. Prior to challenge the response of platelets, from horses with chronic obstructive pulmonary disease (COPD) and normal animals, to adenosine diphosphate (ADP) and platelet activating factor (PAF) did not differ. Five hours after initiation of the challenge, there was a modest but significant decrease in the response of platelets from the COPD horses to PAF, but not to ADP. Platelets from normal horses were not less sensitive to either agonist at this time. Twenty four hours after challenge the responses of platelets from COPD horses to both agonists were the same as pre-challenge values. These results demonstrate that antigen challenge alters the responsiveness of platelets from allergic horses to PAF and this change is suggestive of PAF release accompanying allergen exposure in the horse.