Human exudative neutrophils have greatly increased stores of the neutrophil chemoattractant IL-8 compared with peripheral blood cells, but the mechanism for the increase is not defined. In this report, we show that treatment of peripheral blood neutrophils with the chemotactic peptide fMLP or with leukotriene B(4) or fibrinogen results in little increase in the production of IL-8 by peripheral blood neutrophils. However, a chemotactically active dose of fMLP (5 x 10(-9) M) or leukotriene B(4) (1 x 10(-7) M) in the presence of a physiological concentration (2 mg/ml) of fibrinogen results in a receptor-mediated, pertussis toxin-sensitive, synergistic 30-fold increase in IL-8 synthesis. The levels of IL-8 attained are comparable to those observed in exudative cells. Higher concentrations of fMLP (1 x 10(-7) M) are associated with reduced IL-8 protein synthesis without IL-8 degradation, indicating a sensitive regulatory mechanism for IL-8 production. Treatment of neutrophils with fibrinogen and fMLP resulted in minimal changes in the steady state levels of mRNA for macrophage inflammatory protein-1alpha and -1beta and monocyte chemoattractant protein-1. In contrast, in the presence of fibrinogen, the steady-state level of neutrophil IL-8 mRNA increased 8-fold with 5 x 10(-9) M fMLP but was not decreased with 1 x 10(-7) M fMLP, suggesting that neutrophils are specifically adapted to modulate neutrophil IL-8 synthesis through transcriptional and posttranscriptional mechanisms. The data indicate that fibrinogen can function not only as a substrate in the clotting cascade, but also as an important effector during the evolution of the innate immune response.