Transplantation of bone marrow can give rise to graft-versus-host desease when donor T lymphocytes; mismatched with the host for major histocompatability (MHC) antigens, become sensitized and attack host tissues. However, graft-versus-host desease can also arise between donor and host with compatible MHC antigens but mismatched for a minor histocompatability antigen^1–3. We report here on the occurrence of severe acute graft-versus-host disease in a male patient with acute myeloid leukaemia who had received bone marrow matched for MHC (HLA) antigens from his sister. Strong cytotoxicity of the post-transplantation (that is, donor) lymphocytes against the patient's pretransplantation lymphocytes was found. Thus, the transplanted lymphocytes differed in a non-HLA antigen from the patient. The possible role of this strong cytotoxic minor histocompatability antigen in the development of graft-versus-host disease in man is being evaluated. Furthermore, with the use of cytotoxic T-cell lines, derived from the patient's 6 day effector cells, we are now able to type for it before grafting.