Nine patients with bronchial asthma who had early and late falls in forced expiratory volume in one second (FEV1)(10 minutes and six hours, respectively) after inhalational challenge with specific antigens were studied for the presence of circulating neutrophil chemotactic activity (NCA). NCA was detected during both the early and the late asthmatic responses; the time course of appearance of NCA in the circulation paralleled that of the falls in FEV1. In contrast, five patients with asthma who had early reactions had only a single early peak of NCA, with no further rise for up to 24 hours. The NCA detected during early and late reactions eluted with macromolecules of an estimated molecular weight greater than 500,000 daltons when applied separately to columns of Sephadex G-200. Since high-molecular-weight NCA is believed to be associated with mast cells, these observations support the view that mediators of hypersensitivity are released in both the late and the early asthmatic responses.