We have previously demonstrated that there is a subpopulation of loosely adherent pulmonary mononuclear cells that can be isolated from minced and enzyme-digested lung tissue with a potent capacity to stimulate allogeneic T lymphocyte proliferation. We now demonstrate that these cells are also capable of stimulating an autologous mixed leukocyte reaction (AMLR) and presenting antigen to autologous T lymphocytes. These loosely adherent mononuclear cells (LAM) were more effective than either alveolar macrophages or monocytes as antigen-presenting cells. Depletion of phagocytic or Fc receptor-positive cells from the LAM population enhanced the stimulation of an reaction AMLR while preserving antigen-induced T lymphocyte proliferation. These results indicate that there are nonphagocytic, Fc receptor-negative accessory cells in human lung parenchyma capable of activating resting T cells in an AMLR and supporting antigen-specific T lymphocyte proliferation. The identity of these cells is uncertain, but the data strongly suggest that the cell is not a classical monocyte-derived macrophage. These antigen-presenting cells may be critical in the initiation of immune responses within the lung.