Thirty-eight patients with haematological malignancies were treated with bone marrow transplantation using histocompatible immunotoxin T cell-depleted marrow siblings. All patients received conventional postgraft immunosuppression (methotrexate and/or cyclosporin A). Donor bone marrow was treated ex vivo with T101 Fab fragment coupled to ricin A-chain (T101 Fab-RTA) at a concentration of 10(-8) M of A-chain in association with NH4Cl (2 x 10(-2) M) in pH adjusted (7.8) incubation medium. A median cytoreduction of 99.5% (91-99.5) was obtained. The median of follow-up was 300 days. Only three patients developed grade II acute graft-versus-host disease (GVHD) (actuarial rate of acute GVHD: 9.1%). No chronic GVHD occurred. All patients but one engrafted. Six out of the 37 patients developed a documented bone marrow rejection (actuarial rate of graft failure: 18%). Ten patients relapsed (actuarial rate of relapse: 36.9%). These findings demonstrate that treatment of donor marrow with T101 Fab-RTA in association with NH4Cl at critical pH value can achieve a high level of mature T cell depletion and greatly reduce the incidence of bone marrow rejection and relapse after T cell-depleted allogeneic bone marrow transplantation.