A peptide sequence of the so-called minor H antigen. The minor H antigens are associated with Graft versus Host disease. The peptide and its derivatives find many uses in bone marrow transplantation, organ transplantation, and in the treatment of leukemia. The peptide and its derivatives can be incorporated into vaccines and pharmaceutical formulations, and they can be used in diagnostic test kits. The peptide is derived from the HA-1 minor antigen, and has the sequence VLXDDLLEA (SEQ ID NO: 1), wherein X represents a histidine or arginine residue. Both donors and recipients in bone marrow transplantation can be treated with the peptides, optionally in combination with other peptides, coupled to carriers, and with suitable excipients and/or adjuvants.