Passenger mononuclear cells in organ grafts are known to influence the alloimmune response to the graft. To assess their relevance in clinical lung transplantation, we studied the amount, distribution, cell types, and surface marker expression of mononuclear cells in human donor lungs. Two major compartments of mononuclear cells could be differentiated: lymph nodes containing resting T and B lymphocytes, and the lung tissue itself, containing mainly activated lymphocytes as well as monocytes/macrophages. Tissue-associated mononuclear cells make up 20-40 x 10(9) cells per lung, about 30-50% of which are lymphocytes. Tissue-associated lymphocytes are predominantly T and NK cells; most of the T cells are CD8+ CD45R0+ and express HLA-DR. Strong expression of the adhesion molecules LFA-1 and ICAM-1 is present on infiltrating cells as well as on resident cells of the organ. Moreover, the lymphocytes inside the lung tissue are functionally highly active, with a strong stimulatory as well as alloreactive potency. Thus, large numbers of allogeneic mononuclear cells and particularly large numbers of functionally active lymphocytes are obviously transmitted by human lung allografts. The immunological in vivo relevance of these cells after lung transplantation may include allostimulation and graft-versus-host activity, but also beneficial immunomodulatory effects.