It is generally accepted that the overall recombination frequency between the H–2K and H–2D loci of the mouse major histocompatibility complex (H–2) is at a low level of about 0.4%1. However, the value fluctuates depending upon the combinations of H–2 haplotypes in the heterozygote1. We re-examined the intra-H–2 recombination frequency, using three B10.W congeneic strains which carry the H–2 complex of Japanese wild mice. Of the three, one strain, B10.MOL-SGR(H–2wm7), showed an extremely high incidence of recombination (about 3%) in all combinations with three different inbred B10 congeneic strains. The serological typing of these recombinants revealed that most of the crossing-over points had occurred between the H–2K and IA loci. These results suggest that the wm7 haplotype enhances specific recombination irrespective of H–2 partners in heterozygous parents.