THE Ir genetic region of the mouse which lies between the H-2K and H-2D loci that control the original serologically detectable H-2 specificities, has also been shown to contain genes controlling a new set of serological specificities called Ia for immune associated. Antisera raised by cross immunisation between H-2 recombinant strains have been shown to recognise the products of several genes within the Ir region (reviewed in ref. 1). Unlike the H-2K and H-2D antigens the Ia antigens have a tissue distribution that often does not include T lymphocytes.