We performed immunohistopathologic studies on biopsied lung tissue obtained from two patients with lupus pneumonitis using immunofluorescence, immunoperoxidase, electron microscopy, and acid-microelution. In both patients, immunofluorescence showed granular deposits of IgG, the third component of complement (C3), and DNA in the alveolar walls. The immunoperoxidase technique in both and electron microscopy in one showed that these deposits were in the interstitium of the alveolar walls and in the alveolar capillary walls. The eluates obtained from cryostat sections of the biopsied lungs contained antinuclear factor of IgG class in one patient and showed anti-DNA antibody activity in both. We suggest that the deposits are immune complexes composed of DNA, anti-DNA antibody, and complement and that deposits of DNA-anti-DNA immune complex may play a role in lupus pneumonitis.