The density dependence of maximum flow in ten atopic asthmatics was assessed by response to breathing a helium-oxygen mixture. They were then subjected to bronchial challenge by Dermatophagoides pteronyssinus and density dependence re-assessed during early and late bronchial reactions. There was considerable individual variation in the density dependence before challenge and in the change in density dependence during induced bronchospasm. However, for any given individual the results were similar during early and late reactions. This suggests that the major site of airflow obstruction may vary from one asthmatic to another but that when antigen challenge is followed by a dual early and late reaction the site of obstruction is similar at each stage.