Of patients with collagen disease, anti-lung tissue antibody (ALA) was extensively investigated in relation to the presence of pulmonary fibrosis. Relationship between ALA and other autoantibodies, e.q. rhuematoid factor (RF) and antinuclear factor (ANF) was also studied. From either dog or human lung tissue, crude extracts were obtained and used as antigen, after which ALA was assessed by tanned red cell hemagglutination. ALA was revealed in 16 cases (34.8%) of 46 cases of systemic lupus erythematosus (SLE), 9 cases (64.3%) of 14 cases of progressive systemic sclerosis (PSS), 9 cases (69.2%) of 13 cases of polymyositis plus dermatomyositis (PM-DM) and 10 cases (30.3%) of 33 cases of rheumatoid arthritis (RA). In the control group, only 2 cases (5.1%) of 39 cases ranging from 20 to 80 years old were positive. A significant correlation between the presence of pulmonary fibrosis and the level of ALA was demonstrated in patients with PSS and PM-DM, but not in cases with SLE and RA. ALA, however, did not correlate with the titer of RF and ANF. Antilung serum was prepared by injecting solubilized dog lung antigen into rabbits. The antibody absorbed with kidney antigen is bound to the lung, suggesting the existence of specific antigen in the lung. A linear deposition in the renal glomerulus of -globulin was seen in the lung and kidney of rabbits sensitized with lung antigen. These findings suggested the existence of an antigen common to lung and kidney.