Embryo cell transplantation antigens have been measured by the titration of isoantisera absorbed with allogeneic embryo cells, using a microcytotoxic test. H-2 antigens appear and increase in 9-day embryos at approximately equal rates in two of the three strains examined; results with a third strain suggest the possibility of strain-dependence of transplantation antigen development. The cells of 9-day mouse embryos are considerably less antigenic than adult lymphoid cells.