In a multicenter pilot study, 32 patients showing steroid-resistant acute graft-versus-host disease (GVHD) were treated by in vivo administration of anti-interleukin-2 (IL-2) receptor monoclonal antibody (MoAb B-B10). Twenty-three patients received marrow from HLA-matched related donors, four from matched unrelated donors and five from partially matched related donors. The overall grade of GVHD was II in 16 patients, III in two, and IV in five. Five milligrams of B-B10 MoAb was infused in bolus daily for 10 days and then every second day for a further 10 days in an attempt to reduce GVHD recurrence. No clinical side effects were noted during the B-B10 treatment period. A complete response (CR) acute GVHD was achieved in 21 patients (65.6%). Six patients (18.7%) showed partial improvement (PR) and 5 patients (15.6%) no response (NR). A significant factor associated with GVHD response was the delay between the onset of the GVHD and the first day of B-B10 infusion. The earlier B-B10 was introduced, the greater the probability of CR (P = .03). There was no correlation between the serum B-B10 level and GVHD response (P = .69). There was, however, a significant correlation between the clinical response and the B-B10 kinetics as a function of time: serum B-B10 levels attained a plateau level more rapidly in the CR group than in the PR/NR group. Among the 26 complete and partial evaluable responders, GVHD recurred in 10 cases (38.4%). Host anti-B-B10 MoAb immune response occurred in only one (7.1%) of the 14 patients analyzed. Fourteen of the 32 patients (43.7%) are currently alive between 2 and 14 months after GVHD treatment with B-B10 was completed.