The recognition of alloantigens by human lymphoid cells initiates a collaborative cellular pathway that rather rapidly induces in adherent cells (monocytes) the synthesis and expression of cell surface tissue factor, the initiating cofactor of the extrinsic coagulation pathway. This response was vigorous, generating tissue factor to a level nearly comparable to the response to endotoxin. However, it was temporally discordant with characterized lymphoid procoagulant responses to endotoxin, virus, and immune complexes in that it reached a maxima at 48 h, well after these other responses but clearly much faster than the well recognized proliferative responses to allogeneic stimulation. Using the Daudi lymphoblastoid B cell line, the allogeneic response could be fully elicited in a dose-dependent fashion within 18 h. The induction of monocyte tissue factor required collaboration with T lymphocytes, in accord with previously described T cell-instructed monocyte responses. HLA-DR was implicated as the allogeneic signal by the ability of two monoclonal antibodies to completely block, in a dose-dependent fashion, the induction of this pathway. Notably, the allogeneic procoagulant response was quantitatively discordant with respect to the allogeneic proliferative response, suggesting differences in specificity. This relatively rapid response may be applicable to typing of determinants in the major histocompatibility complex that are not equivalently identified by alternative analyses, and may be significant in tissue transplantation. The cellular pathway, linking allogeneic recognition with induction of a monocyte response that initiates the coagulation pathway, represents a further example of the linkage between these biologic systems, and is consistent with a pathogenetic role in allograft rejection by the promotion of vascular thrombosis and interstitial fibrin accumulation.