Plasma endotoxin, tumor necrosis factor-alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), interleukin 1 receptor antagonist (IL-1ra), and interleukin 6 (IL-6) concentrations in 69 bacteremic patients were compared with those in 54 nonbacteremic patients suffering from suspected bacterial infections. Only three (11%) of the 27 patients with gram-negative bacteremia showed detectable levels of endotoxin. TNF-alpha was detected in 6% of the bacteremic patients and in none of the nonbacteremic patients. Median IL-6 levels were significantly higher in bacteremic than in nonbacteremic patients (55 vs. 0 pg/ml, p = 0.0008). IL-6 concentrations were similar in neutropenic and non-neutropenic bacteremic patients (median 55 vs. 74 pg/ml). In contrast, neutropenic bacteremic patients had significantly lower concentrations of IL-1ra than non-neutropenic bacteremic patients (250 vs. 1,950 pg/ml, p < 0.0001). Patients with fatal bacteremia had significantly higher concentrations of IL-6 and IL-1ra than the survivors (median, 450 vs. 40, p = 0.012 and 7,600 vs. 420 pg/ml, p = 0.0075, respectively). Determinations of endotoxin or TNF-alpha in patients with suspected bacteremia failed to offer clinically relevant data on the prognosis of these patients. IL-6 levels correlated with both the presence of bacteremia and the risk of death. Granulocytopenic patients with bacteremia had lower levels of circulating IL-1ra than patients with normal granulocyte counts, and these levels correlated with poor outcome.