Several authors have observed that the plasma levels of carcinoembryonic antigen (CEA) in patients with neuroblastoma were significantly elevated. The present study was undertaken to investigate the nature of CEA activity in neuroblastoma tissue. This tumor tissue contains a small amount of CEA-like substance reacting with anti-CEA serum which is characterized by gamma-globulin electrophoretic mobility, a molecular weight that is approximately equal to that of albumin (4.6S) by gel filtration, and a glycoprotein staining with periodic acid-Schiff (PAS). According to the double immunodiffusion method, this antigen is partially identical to purified CEA of colon carcinoma, and is completely identical to nonspecific crossreacting antigen (NCA). This antigen is, therefore, referred to not as the CEA as described by Gold, but as NCA in neuroblastoma tissue. The elevation of plasma CEA activity in patients with neuroblastoma may be due to the release of NCA from tumor cells, or to the destruction tissues by metastasis, of normal which are rich in NCA, or to a combination of both.