Uremia has been associated with immunologic aberrations, including anergy, increased susceptibility to infections, and reduced phagocytic activity of polymorphonuclear leukocytes. In this study, cytokine release in uremic and nonuremic blood after in vitro endotoxin stimulation was studied. Blood from nonuremic controls, chronic renal failure patients not on dialysis, and chronic hemodialysis patients predialysis and postdialysis was spiked with 10 ng/mL of Escherichia coli endotoxin and incubated for 2 and 26 h. Plasma tumor necrosis factor-alpha (TNF alpha) concentrations were determined by ELISA after each incubation period. To further study which uremic blood component may be responsible for enhanced release of TNF alpha, plasma and cellular components of chronic renal failure patients and controls were switched and then given an in vitro endotoxin stimulation (1 ng/mL). It was found that (1) TNF alpha release is enhanced by uremia and is exacerbated with progressive declines in renal function, (2) enhanced TNF alpha release is related to a blood cellular phenomenon induced by uremia, and (3) enhanced TNF alpha release in hemodialysis patients is associated with a prolonged stimulation and/or reduced plasma elimination of TNF alpha.