During Gram-negative sepsis, endotoxin lipopolysaccharide (LPS) may activate host inflammatory responses, resulting in the systemic inflammatory response syndrome and the adult respiratory distress syndrome. In cell culture systems, LPS activation of cellular responses may be potentiated by plasma proteins. In the isolated perfused rabbit lung, LPS administration markedly increases the pulmonary hypertensive response to subsequent administration of platelet activating factor (PAF). We examined whether plasma would potentiate the priming effects of LPS in this model. Male New Zealand White rabbits were used in a standard, isolated buffer-perfused rabbit lung preparation, and the pulmonary hypertensive response to 5 nM PAF was measured after 2 h of perfusion with different LPS doses (0, 1, and 10 ng/mL), with and without plasma (10% by volume). In the absence of plasma, 10 ng/mL LPS, but not 1 ng/mL LPS, increased the pulmonary hypertensive response to subsequent administration of 5 nM PAF. However, in the presence of plasma, 1 ng/mL LPS significantly increased the hypertensive response to subsequent administration of 5 nM PAF. We conclude that components of plasma--possibly LPS binding protein and soluble CD14--potentiate the priming effect of endotoxin, resulting in an augmented pulmonary hypertensive response to PAF. Thus, plasma proteins decrease the threshold at which endotoxin primes the lung and may have a critical role in the pathogenesis of endotoxin-induced acute lung injury.