A unique cell surface antigen of a chemically induced rat hepatoma (Zajdela Ascitic Hepatoma, ZAH) has been identified serologically and purified to apparent homogeneity. ZAH cells, when injected subcutaneously or intradermally into syngeneic hosts, elicit a weak humoral antibody response directed against a single cell surface antigen. In spite of prolonged and extensive immunization, the syngeneic anti-tumor antibodies are solely of the IgM class. Antisera from such immunized animals lyse tumor cells in the presence of complement. These observations were utilized in developing methods for the identification and purification of the antigen. It was observed that administration of purified preparations of this antigen confers ZAH-specific tumor immunity in syngeneic animals. Hence the serologically unique antigen is also the tumor-associated transplantation antigen (TATA) of this tumor. This is the first reported purification of a TATA from a rat tumor. There is considerable earlier evidence for the existence of tumor-associated factors which elicit a strong suppressor-cell response in tumor-bearing rats. The availability of the purified TATA from a rat tumor provides, for the first time, a handle for the identification and further characterization of such molecular species.