Human adenocarcinomas of the digestive tract contain common “tumor-specific” antigens (1). These antigens have also been found to cross-react with antigens found in embryonic and fetal tissue but not with normal tissue (2). Sera from pregnant women appear to contain antibodies directed against these antigens (Gold, personal communication). Hamster cells which have been transformed by simian virus 40 (SV40) have a surface (S) antigen which has been found to be specific for cells transformed by the virus (3). This report describes observations that suggest that the surface antigen on SV40-transformed Syrian hamster cells may be immunologically related to an antigen found during embryonic development. Materials and Methods. Immunofluorescence techniques for the detection of SV40 S antigen have been previously described (3, 4). Antisera reacting specifically with the S antigen were prepared by two methods. In the first method weanling hamsters (Con Olsen) were injected with 106 SV40-transformed marmoset monkey cells at weekly intervals for 4 weeks.