Infiltration of mononuclear cell subpopulations was analyzed immunohistochemically in acute lung allograft rejection (BN/LEW) and mycoplasma pulmonis bronchopneumonia in inbred rats. In lung allograft rejection, marked infiltration of activated macrophages, moderate infiltration of helper and suppressor T lymphocytes, and slight or moderate infiltration of B lymphocytes were observed around bronchioles and vessels, as a thick layer of cells like concentric circles, forming a dense cuff. In acute and chronic mycoplasma infection, focal inflammatory reactions were observed with various degrees of infiltration of mononuclear cell subpopulations around bronchioles and vessels, ranging from very mild to severe infiltration, even in the same section or even around the same bronchiole. This difference in the infiltration pattern between the focal, unequal and dappled infiltration in lung infection and the homogeneous and even cuff infiltration in acute rejection may be useful in differential diagnosis between acute rejection and infection following lung transplantation.