Surface antigen (S) common to rat and mouse embryonal carcinoma cells and to pre-implantation embryos.

: Five R rat embryonal carcinomas were induced by inoculating MSV into the placenta of fectotomized rats. Anti-embryonal carcinoma antisera were prepared by allogeneic or xenogeneic immunization with ascitic embryonal carcinoma cells. To remove the non-specific activity both antisera were absorbed in vivo and in vitro. By indirect immunofluorescent assay these absorbed antisera were reactive only on rat embryonal carcinomas and on undifferentiated primitive teratocarcinoma cells of C3H and 129/SV mouse. They did not react with the differentiated cells of mouse teratocarcinomas, with other rat and mouse tumors and with various normal rat and mouse tissues including spermatozoa. A positive reaction was found on mouse and rat pre-implantation embryos from the 4-cell stage to late blastocyst.