T he present study explores the interactions between lymphocytes and monocytes that are required for expression of procoagulant activity (PCA) by monocytes in response to purified protein derivative of the tubercle bacillus (PPD) or tularemia antigen. The PCA response was antigen specific: peripheral blood mononuclear cells (PBM) from donors sensitive to PPD or tularemia showed an increase in PCA only in response to the sensitizing antigen. The PCA was tissue factorlike in that Factors VII and X were required for expression of the activity, whereas Factor VIII was not. Maximum PCA developed only after 36 to 72 h. Fractionation of PBM into lymphocytes and monocytes after antigenic stimulation demonstrated that greater than 90% of the PCA was associated with monocytes. Isolated monocytes or lymphocytes incubated with sensitizing antigen had the same PCA as control cells. Purified lymphocytes that had been pulsed with antigen were unable to elicit a PCA response from monocytes to which they were added. However, adherent monocytes incubated with antigen, then washed free of unbound protein, were able to trigger lymphocytes to become stimulatory for PCA toward responding monocytes. The development of antigen-specific PCA in PBM could be blocked by including a monoclonal antibody to HLA-DR antigen in the incubation. The antibody had no effect on the clotting assay, on preformed PCA, cell viability, or on stimulatory antigen itself. These results indicate that elaboration of PCA by mononuclear cells may be an intrinsic part of the classical immune response to antigen, and may explain the presence of fibrin in immune lesions, as well as the occurrence of thrombotic complications in many immune disorders.