Circulating concentrations of the mast cell-associated mediators, histamine and neutrophil chemotactic factor (NCF) of high molecular weight, were measured in atopic and nonatopic asthmatics after treadmill exercise. Elevations in the concentrations of both mediators accompanied the development of exercise-induced asthma (EIA). Normal individuals did not release mediators or develop bronchoconstriction after an identical exercise. The elaboration of mediators was not due to the onset of airflow obstruction, the postexercise basophilia, or the exercise task per se. A treadmill exercise undertaken while inhaling fully conditioned air inhibited EIA and NCF release; in contrast the same exercise undertaken while breathing cold, dry air elicited EIA and the production of mediators. This suggests that the stimulus for EIA and mediator release may be identical. Late-phase asthmatic reactions occur 3 to 9 hr after exercise in some asthmatics and are accompanied by the appearance of circulating NCF, as previously reported in allergen-induced late responses. In addition to the contribution of mediators to the spasmogenic reaction in EIA, mediators may contribute to bronchial inflammation by activating circulating leukocytes. There was a kinetic increase in the expression of neutrophil C3b receptors in EIA (+) asthmatics for up to 60 min after treadmill exercise. The enhancement of C3b receptors, as evidence of neutrophil activation, was preceded by release of NCF and reductions in peak expiratory flow rates. The prior administration of cromolyn inhibited EIA, NCF release, and enhancement of C3b receptors. These changes were not observed in EIA (-) asthmatics after an identical exercise task. These findings support the view that mediators are released in EIA and may play an important role in its pathogenesis.