Asthma is characterized by the presence of an inflammatory cell infiltrate in the bronchial mucosa consisting of activated mast cells, eosinophils, and T cells. Several cytokines are considered to play a pivotal role in this response, particularly interleukin (IL)-4, IL-5, IL-6, and tumor necrosis factor-alpha (TNF-alpha). In this study, we have used immunohistochemistry applied to thin glycol methacrylate sections of bronchial mucosal biopsies to define the cellular provenance of these cytokines in normal and asthmatic airways. Both the asthmatic and normal mucosa contained numerous cells staining positively for all four cytokines, with the majority identified as mast cells by their tryptase content. Eosinophils also accounted for some IL-5 immunostaining in the asthmatic biopsies. By using two monoclonal antibodies directed to different epitopes of IL-4, we provide tentative evidence for enhanced IL-4 secretion in asthma. Similarly, a sevenfold increase in the number of mast cells staining for TNF-alpha in the asthmatic biopsies suggests that this cytokine is also up-regulated in this disease. These findings clearly identify human mast cells as a source of IL-4, IL-5, IL-6, and TNF-alpha and add to the view that, along with T cells, mast cells may play an important role in initiating and maintaining the inflammatory response in asthma.