A transformation associated antigen was isolated from an SV40 induced hamster sarcoma by sequential silica gel column chromatography and preparative silica gel 60 thin layer chromatography after tissue extraction with chloroform:methanol (2:1, v/v). It migrated with an rf of 0.21 on silica gel 60 thin layer chromatography plates predeveloped and developed in chloroform:methanol:water:glacial acetic acid (10:10:1.5:0.5, v/v) and an rf of 0.27 on cellulose F254 thin layer chromatography plates developed in the same solvent system. Antigenicity was determined using a fluorescence probe cytotoxicity assay to measure inhibition of antibody mediated complement dependent damage to homologous cultured transformed cells. Although compositional analysis of this substance is not complete, it appears to be a polar lipid and would support the concept that transformation associated antigens may be gene plus substrate specific rather than strictly gene specific.