Immune reactions are often associated with fibrin deposition. The mechanisms that lead to this fibrin formation have not yet been clarified. In this study, we show that mononuclear leukocytes (MNL) from donors with a high proliferative response to protein derivative of tuberculin (PPD) have on average 2.5 to 15-fold more thromboplastin (TP) activity, both exposed and intracellularly, compared with MNL from low responder after 4, 7 and 11 days of culture in the presence of PPD. This difference was not observed between the two groups of donors when PPD was omitted from the culture medium. Further evidence for a specific enhancement of the leukocyte TP activity can be derived from the about 4-fold increase in TP activity of the bilateral mixed leukocyte culture relative to the individually cultured controls after 7 days of incubation. TP activity was associated selectively with the monocytes in a PPD-stimulated culture of MNL. The PPD-specific enhancing effect on the leukocyte TP activity could be transferred by soluble factor(s) from stimulated lymphocytes to purified monocytes, which suggests that the effect is, at least partially, mediated by lymphokine(s).