Changes of components of mononuclear cell subpopulations in bronchoalveolar lavage (BAL) fluid were analyzed during acute rejection after lung transplantation in an inbred rat model. All allotransplants (BN/LEW) developed progressive acute rejection from day two until day six, demonstrating severe perivascular and peribronchiolar infiltration of mononuclear cell subpopulations. The number of total cells and activated macrophages increased significantly in BAL fluid from vascular phase until alveolar phase of acute rejection. The number of T and B lymphocytes in BAL fluid increased in alveolar phase, but not in vascular phase of acute rejection. The increase of the number of activated macrophages in BAL fluid may be useful as a monitor of acute allograft rejection following lung transplantation.