Interleukin 6 (IL-6) levels in various materials from patients with sarcoidosis were determined. The subjects of the study were 38 patients with sarcoidosis and 28 healthy controls. For detection of IL-6, an enzyme-linked immunosorbent assay method was used. Interleukin 6 activity in serum was detected in 4 of 30 patients, but not in 19 controls. In bronchoalveolar lavage (BAL) fluid, following 20-fold concentration, IL-6 activity was detected in four of ten patients (nonsmokers) and three of seven controls (two of two smokers and one of five nonsmokers). Interleukin 6 levels in the supernatants of cultured monocytes and alveolar macrophages (AMs) were significantly higher (p < 0.01 and p < 0.01, respectively) in patients with sarcoidosis than in controls. Interleukin 6 production from monocytes tended to correlate with that from AMs. A significant correlation (r = 0.70, p < 0.05) was found between IL-6 production from AMs and the ratio of CD4+/CD8+ in BAL fluid, although no correlation was observed between that from monocytes and CD4+/CD8+ ratio in BAL fluid. Taken together, IL-6 may be involved in the initiation and maintenance of alveolitis by activating and causing the proliferation of T cells.