Graft-versus-host disease (GVHD) is the major complication of allogeneic HLA-identical bone marrow transplantation. GVHD is induced by the activation of mature T cells in the graft which react against minor antigens of the recipient. Mixed epidermal cell-lymphocyte cultures (MELC), which constitute an in vitro model of epidermal cell-lymphocyte interactions, make it possible to study the presentation of antigens to the lymphoid cells by epidermal Langerhans cells. We performed MELC in 66 patients who had received an HLA-identical bone marrow transplant for malignant blood disease. The bone marrow received by 29 recipients had been depleted of mature T cells, whereas 37 recipients had received a non-depleted bone marrow. A complete, uni- and multivariate statistical analysis was carried out on recipients of non-depleted bone marrow to evaluate the risk factors for acute and chronic GVHD. This study showed that MELC between donor and recipient was the most predictive parameter for the occurrence of GVHD. Other factors were a history of previous pregnancies in female donors and a diagnosis of chronic myelogenous leukaemia. These results may be of value in the selection of donors and for a better determination of the need for bone marrow depletion.