DSHNHL R3 is a randomized clinical phase II study. The main objective is to estimate the efficacy of rituximab as a prophylactic medication for prevention of graft-versus-host-disease after allogeneic peripheral stem cell transplantation in patients with a high risk relapse of aggressive B-cell Non-Hodgkin's lymphoma. The most important secondary objective is to estimate the efficacy of allogeneic stem cell transplantation in this clinical situation.