After bone marrow grafting, severe unexplained thrombocytopenia and granulocytopenia may complicate the post-graft recovery of the patient. The present study has shown the presence of antibodies to platelets and granulocytes of donor origin in recipients of both allogeneic and autologous bone marrow grafts. In the case of autografts, such antibodies are by definition autoantibodies, and similar antibodies after allografting may also have an autoimmune origin. It is likely that this is the result of transient immune system imbalance, common to both allo- and autografts, in the early post-graft period. The extent to which these antibodies affect the peripheral counts probably depends on the ability of the engrafted marrow to compensate for the rate of antibody-mediated cell destruction.