We report on a male who received a bone-marrow allograft from his HLA identical sister for acute myelogenous leukaemia. After transplantation, the patient suffered from refractoriness to the transfusions of HLA-matched platelets and a strong platelet-specific antibody, anti-HPA-2b, of IgG1 subclass was demonstrated in the patient's serum. In the serum of the bone-marrow donor a weak IgG1 anti-HPA-2b was demonstrated. IgG allotyping of the patient and donor showed identical results. We could not determine the origin of the anti-HPA-2b, although we hypothesize that the anti-HPA-2b was produced by immunocompetent donor lymphocytes infused with the suspension of bone-marrow cells.