In vitro studies have suggested that basophils are involved in the asthmatic response to isocyanate. Classified by the type of airway response, three groups of patients exposed to isocyanate were studied: 1) nonreactive (n=3), 2) immediate responders (n=4), and 3) late responders (n=4). Basophil histamine release to concanavalin A (conA), FMLP, and anti-IgE was evaluated before and during the airway response. Histamine release significantly (P=0.009 for conA and 0.03 for anti-IgE) increased after exposure to isocyanate only in the group which had late bronchial response. Additionally, good correlation was found between baseline FEV1FVC% and degree of histamine release of basophils stimulated with conA. These findings support a role for basophils in the late asthmatic response to isocyanate.