We have cloned six different class I genes from a B10.P sperm library. After cotransfection with the herpes simplex tk gene, one L-cell line was found to react with six H-2Dp-specific monoclonal antibodies. The cell line L12a did not react with Kp-specific monoclonal antibodies. This identification was confirmed by mapping a 2.5 kb Bam H1 restriction fragment present in the lambda 12a DNA clone to the D-TL region of H-2p. Only a single 8.8 kb Bam H1 fragment can be assigned to Kp by restriction fragment length polymorphism, while many others map to the D-TL interval. A restriction map of lambda 12a is presented.