Airway wall remodelling is an established pathological feature of asthma but its causes are not well understood. One cytokine of potential relevance is transforming growth factor-beta1 (TGF-beta 1). The immunolocalization of TGF-beta 1 and of its small binding proteoglycan decorin have been examined in the airways of normal subjects and atopic asthmatics. Bronchial biopsy specimens were obtained by fibreoptic bronchoscopy, processed into glycolmethacrylate resin, and stained immunohistochemically using specific antibodies. Immunoreactive TGF-beta 1 was principally localized extracellularly in association with subepithelial connective tissue. Some staining of bronchial epithelial cells was also evident, but otherwise there was little intracellular staining. The overall pattern of immunohistochemical staining was indistinguishable in biopsy specimens from asthmatic and control subjects. Comparison of adjacent sections demonstrated the co-localization of immunoreactivity for TGF-beta 1 and decorin in the mucosa. It is concluded that immunoreactive TGF-beta 1 in human airways is principally extracellular and that matrix-associated TGF-beta 1 is likely to be bound at least in part to decorin. This interaction may provide a reservoir of TGF-beta 1 that can be released in an active form in response to appropriate stimuli.