Immunohistochemical analysis was performed on skin biopsies from patients suffering from graft versus host disease (GVHD) following bone marrow transplantation for aplastic anaemia and leukaemia. Lymphoid cells infiltrating the skin were exclusively T cells with the OKT8+ phenotype and these are probably cytotoxic T cells. Langerhans cells were reduced in number in all specimens and a variable number of HLA-DR positive macrophages were seen in the dermis. In several cases HLA-DR antigens were expressed on keratinocytes. These results show consistent features which may help discriminate rashes in the skin in the post-transplant period.