Lung macrophages may play an important role in the pathogenesis of pulmonary sarcoidosis. In this study, the ability of pulmonary macrophages and blood monocytes from sarcoidosis patients, normal controls and disease controls to provide the accessory signal necessary for the concanavalin A-induced activation of normal blood T cells was examined. Blood monocytes from all groups supplied a significantly greater accessory signal than lung macrophages. The accessory capacity of lavage macrophages from sarcoidosis patients varied over a wide range and correlations were sought between these values and other parameters of disease activity. Whilst there was no correlation with clinical parameters, accessory function of alveolar macrophages correlated significantly with the percentage of T helper cells in bronchoalveolar lavage (BAL) fluid (p less than 0.05) and, more closely, with the T helper:T suppressor ratio in BAL fluid (p less than 0.01). This interrelationship between macrophage activity and the T cell infiltrate favours the probability that both cell types participate in the sarcoid disease process and raises the possibility that T cells of both helper and suppressor phenotypes contribute to the pathogenesis.