The H-2dm1 mutation is unique among all described H-2 mutations in that two transplantation antigens, the H-2Dd and the H-2Ld, are affected. Here, we show that the mutant gene, Ddm1, is formed by fusion of the 5' part of the Dd gene and the 3' part of the Ld gene, with the region in between deleted. The recombination junction is located in the third exon, which encodes the alpha 2 region of the protein. When the hybrid gene is transfected into mouse L cells, serological and biochemical analyses indicate the Ddm1 antigen expressed in the transformant line is identical to the mutant molecule in dm1 spleen cells. These results demonstrate that the D/L hybrid gene is most likely responsible for the dm1 mutant phenotype.