SummaryGoat and rabbit antisera were prepared against the fraction soluble in 50% saturated ammonium sulfate made from saline extracts of bronchogenic carcinoma, hepatoma, and colon carcinoma metastatic to the liver. The antiserum, after being absorbed with nonmalignant adult tissues, was reactive not only with lung cancer but also with other cancers and fetal tissues. By immunodiffusion, two tissue components designated X and Y were prominent, at least quantitatively, for lung cancers. Of 12 patients, 5 had both antigens, 6 had only Y antigens, and 1 had none. Both antigens, including their related components, were present in carcinomas of the stomach, pancreas, liver, and kidney. Some gastric and colonic cancers had a different antigenic composition from lung cancers. Immunoelectrophoresis showed multiple components and these antigens were considered to be complexes. There was a reaction of partial identity between the Y antigen and fetal protein; the antigenic determinants were greater in the Y antigen than in fetal protein. Gel filtration indicated that these antigens were larger in molecular size than serum IgG and that the X antigen was larger than the Y antigen. Other partial characterization studies were performed. Preliminary immunofluorescent study revealed that significant positive staining was observed for cancer cells but not for nonmalignant portions of tissues, which suggests a possible application to diagnosis.