As part of an ongoing phase-I/II trial, 2 patients received a 5-day treatment course with a murine monoclonal antibody (MAB) directed against the human T cell receptor (BMA031) as primary therapy of acute grade III skin and gastro-intestinal graft-versus-host disease (GvHD) occurring after allogeneic bone marrow transplantation (BMT). All MAB infusions were tolerated without side effects. A complete response of all symptoms of acute GvHD could be attained by MAB therapy under a continued baseline immunosuppression with cyclosporin (CSP), and both patients remain alive and disease-free at 7 and 8 months after therapy without evidence of chronic GvHD. Although the exact treatment scheme has still to be defined, we conclude that this MAB may be useful as primary therapy of acute GvHD. However, the potential hazards of 'in vivo' therapy with MABs directed against T lymphocytes call for a critical evaluation of this treatment modality.