The intravenous injection of endotoxin in human plasma into rabbits produces a marked potentiation of lethal effect, when compared to the mortality associated with comparable doses of endotoxin in saline alone. A similar enhancement was noted with other heterologous plasma (guinea pig, rat) but not with homologous plasma. The potentiating factor is not in the albumin or gamma globulin fractions, is not concerned with fibrinogen, and is heat-labile. Tolerance of the recipient animal to endotoxin destroys the lethal effect. Within a period of 3 hours, endotoxin and human plasma may be administered separately, without regard to timing or sequence, without loss of the lethal effect. The enhancement of lethal effect is not avoided by pretreatment with heparin or cortisone. Preliminary experiments indicate that the loss of lethal effect found after incubation of endotoxin-plasma mixtures may be due to a separate inhibitor, but is not due to loss of the potentiating factor.

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