Two cell surface antigens on rat neural tumor cells are defined by antisera from mice immunized with a rat glioma cell line, 33B. The Common antigen is on rat brain and embryo, and is strongly expressed on the surface of all, or most, rat glioma and neuroblastoma cell lines and tumors. The other Restricted antigen is not present at detectable levels on normal rat tissues, but is on 33B, and on 11 other rat neural tumors or cell lines developed from such tumors, though many other tumors are negative. These 2 antigens are on cell membrane preparations from cells and tumors, and have been further characterized using a quantitative antigen assay. Both antigens are heat labile, and can be destroyed by digestion with proteolytic enzymes. The Common antigen is 10 times more sensitive than the restricted antigen to pronase digestion. Furthermore, spacially separate sites for the 2 antigens are indicated by blocking experiments with pepsin digested antisera. Attempts to purify these antigens further have been frustrated by loss of antigenic activity upon detergent-induced release from the membrane. The tissue and tumor distributions of recently described mouse and rat surface antigens are reviewed. Many of these antigens are present on both brain and kidney, but not on other tissues, though several are shared with embryonic cells or sperm. Several new antigens have been described which may be neuronal specific.