Graft-versus-host disease is a severe complication which may occur even after bone marrow transplantation between HLA identical siblings. Minor transplantation antigens have been implicated as a causal factor. Therefore the influence of the monocyte/granulocyte antigen 9a on GvHD was studied. No influence on GvHD was observed, but absence of the 9a antigen in the donor was associated with an early death of the recipient.