Fibrin deposits were observed in the involved lymph nodes and/or spleens of 15 patients with Hodgkin's disease by specific immunofluorescence and by electron microscopy. Two basic patterns of fibrin deposition were observed: 1) intercellular deposits, chiefly associated with nonneoplastic-appearing lymphoid cells and 2) deposits associated with the collagen fibers of young connective tissue. In addition, coarse fibrin deposits were observed in areas of necrosis, presumably a non-specific finding. Fibronectin was also observed in intercellular areas, but staining was less intense than for fibrin. Fibrin deposits were also observed in 3 of 6 cases of non-Hodgkin's lymphoma, indicating that the finding is not an exclusive feature of Hodgkin's disease. The pathogenesis and possible significance of fibrin deposition in Hodgkin's disease are related to earlier observations of activation of the coagulation system on neoplasia and cell-mediated immunity and to the possible role of fibrin, fibronectin, and their breakdown products in angiogenesis and fibroplasia.