Proinflammatory cytokines are important mediators during endotoxemia. In experimental models, injection of lipopolysaccharide (LPS) activates macrophages leading to excessive secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta and IL-6; infusion of high dose of these mediators results in organ failure and death. Natural infection may be different, because it persists over days or even weeks, with repeated endotoxin challenge to macrophages. Little is known about the capacity of peripheral blood mononuclear cells (PBMCs) to release proinflammatory cytokines under these conditions. Therefore, as an ex vivo model of sepsis, the expression of proinflammatory cytokines after stimulation of whole blood with LPS was studied. A high LPS dose (1 microgram/ml) maximally increased TNF-alpha, IL-1 beta and IL-6 secretion in controls, but a marked depression was observed in septic patients (p < 0.01; 15 patients with severe sepsis versus 20 control patients without infection). This reduction persisted for up to 10 days after diagnosis of sepsis. The release of TNF-alpha, IL-1 beta and IL-6 was markedly decreased in the septic group even when a lower and physiologically more relevant LPS concentration (1 ng/ml) was used. IL-1 beta mRNA was similar to controls, but a down-regulation was observed in TNF-alpha and IL-6 transcript levels in PBMCs from the blood of septic patients. This was at least in part due to a marked reduction in TNF and IL-6 mRNA half-life. These results indicate that different mechanisms down-regulate proinflammatory cytokine release in the whole blood of septic patients. Although excessive secretion is known to be deleterious, low concentrations of these cytokines are involved in regulating essential cellular and humoral immune functions. Thus, the reduced capacity to express and release adequate amounts of proinflammatory cytokines after exposure to endotoxin, as observed in whole-blood PBMCs from septic patients, may contribute to the development of immunodeficiency.