The five H-2t haplotypes are all intra-H-2 recombinants which have identical K and D regions, but differ in the central regions of the complex. Analyses of H-2, Ia and Ss-Slp types indicate that the crossover in H-2t1 took place between the K and I regions, in the H-2t2 between G and D, in H-2t3 and H-2t4 between the I-B and I-C subregions and in H-2t5 between I and S. The set of recombinants will be very valuable for studies of traits which map in the I and S regions.