Primary human adult lung epithelial cells (ALEC) were established in culture using the most distal parts of the lung to avoid the airways. Immunocytochemical peroxidase staining and semiquantitative flow cytometry were used to characterize the cells in conjunction with a panel of monoclonal antibodies (mAb). The cells showed a constitutive expression of major histocompatibility complex (MHC) class I antigens, patchy expression of intercellular adhesion molecule-1 (ICAM-1) and a weak patchy expression of MHC class II antigens (detected using immunocytochemical staining). Incubation of the primary ALEC with interferon-gamma (IFN-gamma) (250 U/ml) stimulated an up-regulation of the expression of these three antigens to varying degrees; expression of MHC class I antigens and ICAM-1 molecules showed an up-regulation at 10 hr after the start of the treatment, reaching a peak at 48 hr, maintaining it for the next 24 hr and then, steadily and progressively, losing it towards the end of the experiment at 96 hr. Expression of HLA-DR showed an up-regulation at 17 hr after the start of the treatment, reaching a peak at 72 hr and maintaining it for the next 24 hr. Cytomegalovirus (CMV) infection of ALEC in culture caused an up-regulation of expression of class I antigens and ICAM-1, but not DR. However, when the infected cells were incubated with IFN-gamma, an up-regulation in the expression of DR took place. Therefore, within the micro-environment of the transplanted lung the presence of cytokines (IFN-gamma) produced by infiltrating activated mononuclear cells, may render the lung epithelial cells capable of acting as antigen-presenting cells, expressing high levels of class I antigens, ICAM-1 and class II antigens, activating CD8 and CD4 cells thus playing a major part in the process of rejection of the lung allograft; themselves becoming a primary target in the process.