Two iso-antigens of the mouse, not related to any known histocompatibility system, are described as H-5.A and H-6.A. H-5.A is present on the red cells, kidney, testis, and lung of C3H/St mice and in smaller amounts on spleen, brain, and muscle. It has been detected on the red cells of 10 out of 29 sublines tested. H-6.A has been found on red cells, testis, brain, and spleen, and in smaller amounts on other organs of C3H/He mice. The tissue distribution appears to be roughly similar in C57 and A mice. The antigen has been found on the cells of 13 strains. Both antigens appear to behave as a single unit, and neither has the same strain or tissue distribution as any previously described histocompatibility factor.