The in vitro secretion of fibronectin by rat alveolar macrophages recovered following the intratracheal instillation of various mineral dusts was examined using a competitive enzyme-linked immunoassay (CELIA) method. Cells derived with the fibrogenic dusts DQ12 quartz and UICC crocidolite asbestos had elevated rates of fibronectin secretion when compared ith those derived from titanium dioxide or saline. The in vitro culture of alveolar macrophages with dusts did not lead to elevated rates of fibronectin secretion, suggesting that mechanisms other than the direct interaction between dusts and macrophages may be responsible for elevated rates of fibronectin secretion by cells exposed to fibrogenic dusts. This suggests that fibronectin deposition seen in pneumoconiotic lesions in immunohistochemical studies may in part have been derived from macrophages. Thioglycollate-induced activated mouse peritoneal macrophages secreted significantly less fibronectin than resident peritoneal macrophages, a finding contrasting with those of Tsukamoto et al. [7].