Partially purified hydrosoluble extracts were obtained by isoelectrofocusing from the membranes of RC 19 tumor cells carried in ascites form in Balb c mice. The biological activity was assayed in vitro by the capacity of the partially purified antigen to absorb cytotoxic activity from allogeneic antisera specific for the FMR tumor associated antigen (TAA) as well as for antisera specific for normal H- 2 transplantation antigens. The partially purified material contained both TAA and normal H- 2 antigen. No separation of normal and tumor antigens was observed at this stage of purification by the methods used. Separation on acrylamide gel revealed two protein bands the identities of which are currently being investigated. The results and their implications are discussed in terms of the physicochemical technique used and the nature of the cell surface antigen.