We have previously reported increased neutrophil chemotactic activity in sera obtained after positive antigen inhalation responses in atopic subjects. This report describes the kinetics of appearance of this serum activity and the effects of antigen dose and disodium cromoglycate pretreatment on the response in 10 ragweed-sensitive subjects. Significantly increased chemotactic activity was present as early as 1 min, peaked at 10 min, and persisted through 24 hr after inhalation of antigen. The increased chemotactic activity correlated with the degree of bronchospasm induced by antigen inhalation and the amount of antigen administered. The increased chemotactic activity and bronchospasm were blocked by administration of disodium cromoglycate prior to antigen challenge. These findings are consistent with a postulated antigen-induced anaphylactic release of chemotactic activity. The correlation of this activity with the degree of bronchospasm and its appearance after administration of even small doses of antigen suggest that this activity may be important in antigen-mediated bronchospasm.