Using a combination of classical genetics, serology, and molecular genetics, we have mapped as many as possible of the MHC components in t haplotypes. Results indicate that the whole MHC is included in a simple inversion with the relative positions of its genes being intact. Furthermore, some of the t-lethal mutations map very near to, or are intermingled with, components of the MHC. At the centromeric end the t12 lethal maps between TL and H-2D in the Qa region, and distally the tw5 lethal is inseparable from H-2K.