Abstract: We disclose the discovery and purification of a tumor antigen that binds the mouse monoclonal antibody L6. Immunohistochemical analysis showed that the antigen is expressed on the membrane surface of LX-1 human lung tumor cells, which retain L6 binding activity when intracerebrally xenografted in nude rats. The antigen is also expressed in the cytoplasm of hypothalamic neurons. Inability of oxytocin and vasopressin hormones to block L6 binding showed that the antigenic epitope resides in neurophysin, the carrier protein associated with the two hormones. Porcine neurophysin did block L6 binding to the antigen. Western blot analysis confirmed that L6-immunoreactivity is neurophysin-related. Immunoaffinity chromatography and gel electrophoresis revealed the antigen molecular weight to be 45000 daltons. Amino-terminal sequencing revealed a 21-amino acid homology with the N-terminus of human pro-pressophysin.