To determine whether neutrophils are activated after exercise-induced asthma, increases in neutrophil complement receptor numbers (complement receptor enhancement) were measured by the rosette technique. In twelve asthmatic patients there was a time-dependent rise in complement receptor numbers for up to 60 min after treadmill exercise. This enhancement of complement receptors was preceded by a rise in plasma neutrophil chemotactic activity and a reduction in the peak expiratory flow rate. These changes could be inhibited by prior administration of disodium cromoglycate. The changes were not observed in seven asthmatic patients in whom asthma was not induced by an identical exercise task. Complement receptor enhancement was also observed in vitro when partially purified neutrophil chemotactic activity from a patient with exercise-induced asthma was incubated with normal neutrophils. These findings suggest that inflammatory cells are activated after exercise-induced asthma, possibly as a result of the release of neutrophil chemotactic activity and other mast-cell-associated mediators.