Histamine has been suggested as an important mediator of the cardiovascular abnormalities during septic shock. To determine if blood histamine levels were increased during human sepsis and septic shock, plasma histamine was measured using a very sensitive radioenzyme assay employing histamine N-methyltransferase (HNMT) in the following patient groups: normal controls (n = 76), nonseptic critically ill (n = 12), nonseptic shock (n = 2), sepsis without shock (n = 28), and septic shock (n = 41). Using this enzyme binding assay, all these groups had similar, normal plasma histamine concentrations, except those patients with septic shock whose mean histamine measurements were significantly reduced (p< .002). This decrease was found to be due to an artifact of the assay: plasma contained a circulating inhibitor that falsely lowered the measured histamine level. Fractionation of septic shock plasma using molecular exclusion membranes and gel filtration revealed a 5000 MW inhibitory factor. After removal of this inhibitor from plasma, septic shock plasma histamine levels were normal. Thus, septic shock patients may have a circulating inhibitor of the HNMT enzyme, but plasma histamine concentrations are normal. Histaminemia is unlikely to play an important role in the pathogenesis of septic shock in humans.