The products of the blood clotting reaction, eg, thrombin and fibrinopeptides, have various proinflammatory activities and are suggested to modulate inflammation. The macrophage expression of tissue factor (TF), the clotting initiator, has been shown to cause clotting in the site of the delayed-type hypersensitivity reaction, a cellular immune response. However, the mechanism of the clotting induction in humoral immune response has been insufficiently studied. Therefore, the Arthus reaction, a model of immune-complex diseases, was produced in monkey skin that was examined for TF expression and fibrin deposition. TF antigen was positive on most of polymorphonuclear leukocytes, which were the main leukocytes in the lesions and were identified as neutrophils with an anti-neutrophil-elastase mAb. TF mRNA was detected in neutrophils by in situ hybridization using TF RNA probes, indicating de novo TF synthesis by the leukocytes. Specific binding of activated factor VII onto TF-positive neutrophils suggested the activity of neutrophil TF to trigger the cascade reaction of clotting. The number of TF-positive neutrophils were correlated in time with the intensity and extent of fibrin deposition that was visualized with an mAb specific for fibrin and peaked in 24 hours. Interestingly, the fibrin deposit was partially positive for an mAb specific for neutrophil elastase-digested fibrin. These results show in vivo evidence of a close relationship between neutrophils and both clotting and fibrinolysis in the Arthus reaction and may suggest that these neutrophil functions contribute to the pathogenesis of this hypersensitivity inflammation.