Tumors of the central and peripheral nervous system were induced in rats with ethylnitrosourea. Many of these tumors were transplanted in syngeneic recipients, and several cell lines were derived from them. An antiserum raised against one such cell line in C3H mice defined two cell surface antigens in cytotoxicity tests. One, the common antigen, was present on rat brain and embryonic tissues and was present in large amounts on most tumors or cell lines from the nervous system. Fibroblastic cell lines had smaller amounts of this antigen, which also could be detected by immunofluorescence. The other, restricted antigen was not detected on normal or other, restricted antigen was not detected on normal or embryonic tissues. It was present on six tumors from the nervous system, on one glial cell line, and on a Schwann-cell line RN22. In addition, it was present on four out of eleven cloned cell lines isolated from rat tumors at the Salk Institute. Two of the positive clonal lines had been shown to have properties unique to neuronal cells. The restricted antigen was therefore expressed on the cell surface of some, but not all, glial, Schwann, and neuronal neoplastic cells.