To characterize the airway responses to inhaled Ascaris suum antigen in a group of adult Beagle dogs with cutaneous Ascaris sensitivity, 9 dogs were challenged with aerosolized Ascaris antigen and 6 were subsequently challenged 1 to 2 weeks later with aerosolized saline. Changes in lung resistance, dynamic lung compliance, and responsiveness to aerosolized histamine were measured during the ensuing 6 hours. Inhalation of Ascaris antigen produced immediate bronchoconstriction, but no spontaneous late bronchoconstrictive responses were noted. Two dogs demonstrated increased histamine responsiveness 6 hours after Ascaris challenge, and 4 dogs were histamine hyperresponsive 1 to 2 weeks later. We conclude that exposure to Ascaris antigen in naturally sensitized Beagle dogs produces an immediate asthmatic response and may result in airway hyperresponsiveness for several weeks. However, spontaneous late responses do not occur.