Tumor necrosis factor (TNF-alpha) and interleukin-1 (IL-1 beta) are cytokines primarily produced by monocytes/macrophages when stimulated by endotoxin, complement-derived anaphylatoxins and the specific antigen. In the present study, the plasma levels of TNF-alpha and IL-1 beta were evaluated before and after hemodialysis with cuprophane membrane (in 9 patients) and hemodiafiltration (in 9 patients) using three high-permeability membranes such as polymethylmethacrylate, polyacrylonitrile (AN-69) and polysulfone. In vitro spontaneous production of TNF-alpha and IL-1 beta was evaluated in the supernatants from short-term cultured monocytes obtained before and after treatment. The predialytic levels of TNF-alpha and IL-1 beta were significantly higher (p less than 0.05) in the uremic population than in 21 healthy subjects taken as controls. The analysis of the uremic population regarding the mode of therapy indicated that in hemodialysis the predialytic plasma levels of TNF-alpha and IL-1 beta did not significantly differ from those of healthy subjects. In contrast, in hemodiafiltration with polymethylmethacrylate and AN-69, but not with polysulfone, the predialytic plasma levels of both cytokines were significantly (p less than 0.05) increased. No significant variation in plasma levels of both cytokines was observed after hemodialysis with cuprophane membranes. Hemodiafiltration with polymethylmethacrylate and AN-69, but not with polysulfone, brought about a consistent reduction in plasma levels of both cytokines. Detectable amounts of TNF-alpha and IL-1 beta were spontaneously produced by peripheral-blood monocytes 6 h after the end of hemodialysis but not of hemodiafiltration. These studies suggest a possible role of TNF-alpha and IL-1 beta in the biocompatibility of different extracorporeal treatments.