Summary.-The tumour specific transplantation antigen (TSTA) from a chemically induced rat sarcoma has been isolated as an electrophoretically homogeneous soluble material by affinity chromatography using a Sepharose bound antibody raised to the tumour in syngeneic rats. The TSTA is specific for the particular tumour used (the MC-1 sarcoma) and does not cross-react with material extracted from other rat sarcomata. In addition, a material with different physicochemical properties which cross-reacted with different sarcomata was also eluted from the antibody column and this may be the previously identified onco-embryonic antigen (OEAl) which is immunogenic in the syngeneic host.The purified TSTA labelled with 125I was used in a radioimmunoassay which detected soluble TSTA in rats bearing a MC-1 sarcoma. The assay shows that tumour transplantation is associated with a persisting release of soluble antigen into the circulation. This antigenic burden is present continuously and renewed as long as the tumour mass exists.