Previous studies indicated that red blood cells and spleen cells of newborn A, C3H and CBA mice were resistant to humoral H-2 antibodies during the first 3 days after birth. It was demonstrated in the present work that this difference in timing with regard to the phenotypic expression of the various H-2 components was preserved when they were brought together on the same F1 hybrid erythrocytes or spleen cells.
It was demonstrated by the indirect fluorescent antibody technique that H-2 antigens were present on all spleen cells of newborn mice, but the frequency of positively stained cells diminished rapidly with decreasing age in embryos and no antigenic cells could be detected in 15-day-old embryos.
X-irradiated cells of 13- to 14-day-old embryos were found to be antigenic, as judged by their capacity to induce transplantation immunity against skin grafts and by their ability to stimulate the formation of humoral antibodies.
It is suggested that the difference with regard to the time of phenotypic expression of H-2 isoantigens as shown by the various techniques is due to differences in the relative sensitivity of the methods used rather than to differences in the kind of isoantigens detected by the different techniques.