The antigens present in a gastric tumor obtained from a patient of blood group p,O,Le(a-,b+) were studied. The neutral glycolipids were isolated from the cancer tissues and characterized chemically and immunologically. The glycolipid pattern of the tumor was similar to that of the surrounding univolved mucosae. The major neutral glycolipids were found to be glucosylceramide, galactosylceramide, lactosylceramide, and the lacto series glycolipids, including the H, Lea, and Leb active fucolipids. The cancer tissues and the uninvolved mucosae did not contain galabio-sylceramide, globotriaosylceramide, or globotetraosylceramide after separations by both thin layer chromatography and gas-liquid chromatography. Thin layer chromatography immunostaining was performed to detect incompatible blood group antigens. Immunostaining revealed the presence of globotriaosylceramide, globotetraosylceramide, Forssman glycolipid, and A active glycolipids in the cancer tissues. These incompatible blood group active glycolipids were absent from the uninvolved mucosae. The results indicate that the cancer tissues possess the ability to produce the globo series of glycolipids and the A active antigens but that the surrounding uninvolved mucosae could not synthesize these glycolipids.