Six patients (five children < or = 12 years old and one young adult) underwent allogeneic BMT (not T lymphocyte-depleted) from sex-matched HLA-identical siblings. GVHD prophylaxis consisted of methylprednisolone (30 mg/m2) and anti-pan T lymphocyte ricin A chain immunotoxin (H65-RTA) (0.1 mg/kg) administered daily for 12 consecutive doses. H65-RTA was initiated at day +5 (n = 4) or day +2 (n = 2). All patients engrafted. Despite receiving the planned GVHD prophylaxis, all patients developed moderate to severe acute GVHD; five patients developed Grade III/IV GVHD. Four patients died 34 to 78 days post-transplant; GVHD was a contributory cause of death in each case. H65-RTA as used in this study was ineffective for the prophylaxis of acute GVHD.