In this study, a combination of two antibodies both conjugated to a cell-killing toxin (so-called immunotoxins) will be evaluated. The antibodies are directed against T-cell antigens 'cluster of differentiation 3 antigen' (CD3) and CD7. Previous in vitro studies have demonstrated that this particular immunotoxin-combination, named T-Guard, acts synergistically in eliminating T cells with a preference for killing activated T-cells. In a subsequent clinical pilot-study, T-Guard has generated encouraging results when applied as third-line therapy for patients suffering form steroid-resistant acute Graft-versus-Host Disease (GVHD). Extensive biological and clinical responses could be noted in the absence of severe acute toxicities. Building on these results, the current study aims at evaluating the safety and efficacy of T-Guard for treating steroid-resistant GVHD when administered in an earlier phase of the disease process, i.e. as second-line instead of as third-line therapy.