We have applied the technique of sputum induction by hypertonic saline in asthmatics and nonatopic control subjects to study an array of indices of airway inflammation believed to be relevant to asthma pathogenesis. Compatible with a central role for eosinophils and mast cells in asthma, sputum of asthmatic subjects contained increased numbers of eosinophils and levels of eosinophil cationic protein (ECP) and mast cell tryptase. Eosinophil numbers, and ECP and histamine levels correlated with the degree of methacholine airways responsiveness, and ECP, tryptase, and histamine correlated with raised concentrations of albumin. Using the micro-Boyden chamber technique eosinophil chemotactic activity was identified only in the sputum from asthmatics. The correlation between the raised levels of total IgA, IL-8/IgA complexes, and tryptase and the degree of sputum eosinophilia and ECP levels, suggests possible mechanisms for eosinophil chemotaxis and activation in asthma. Row cytometric analysis of sputum lymphocytes showed an increase in CD4+ T cells and T cells expressing intercellular adhesion molecule-1 (ICAM-1) in asthma which, together with the finding of raised levels of soluble ICAM-1 in the sputum, indicates upregulation of this adhesion molecule. Finally, the proportion of CD16+ natural killer (NK) cells was reduced in the sputum of asthmatics. These observations highlight the importance of the airway inflammation in causing asthma and further confirm the usefulness of sputum induction as a tool in asthma research.