The concentration and accessibility of endotoxin can increase following antibiotic killing of gram-negative bacteria. There are indications that antibiotics may differ in this respect. We measured endotoxin levels in RPMI 1640 and tumor necrosis factor alpha (TNF-alpha) and interleukin-6 production in whole blood ex vivo after exposure of log-phase Escherichia coli to antibiotics belonging to different classes, in a final concentration of 0.5, 5, or 50 times the MIC. After 4 h of incubation at 50 times the MIC, ceftazidime and ciprofloxacin treatment resulted in levels of endotoxin, TNF-alpha, and interleukin-6 significantly higher than those of imipenem and gentamicin (P < 0.001). Similar differences in cytokine induction were measured after 8 h of incubation. At 0.5 times the MIC, the differences between the antibiotics in measured endotoxin and cytokine levels were small, with levels comparable to the levels in untreated cultures. Polymyxin B and, to a lesser degree, recombinant bactericidal/permeability-increasing protein 21 (rBPI-21) were found to be potent inhibitors of TNF-alpha release, supporting the concept that the differences between the antibiotics in cytokine production were indeed due to differences in amounts of biologically active endotoxin. The presence of serum from patients suffering from untreated sepsis decreased TNF-alpha production significantly, in a concentration-dependent manner.