Hematopoietic progenitor and stem cells are contained within the CD34+ cellular compartment of the bone marrow. Positively selected cytokine primed peripheral blood derived CD34+ cells have been shown to support autologous hematopoiesis after myeloablative therapy. We investigated hematologic reconstitution and incidence of graft-versus-host disease (GVHD) after transplantation of allogeneic peripheral blood CD34+ cells. CD34+ cells were selected from the peripheral blood of 10 matched related donors after treatment with rG-CSF followed by one to four apheresis procedures and biotin-avidin immune affinity purification. Ten patients with advanced hematologic malignancies were subsequently transplanted with cryopreserved allogeneic CD34+ cells after myeloablative chemotherapy. Immune affinity purification of CD34+ cells resulted in a 370-fold T cell reduction. Patients were grafted with a median number of 4.1 x 10(6) kg (1.6-6.4) CD34+ cells and 0.42 x 10(6)/kg (0.29-2.2) CD3+ cells. All patients received rG-CSF 5 micrograms/kg post-transplant and completely engrafted with neutrophils > 500/microliter after a median time of 10 days (9-15) and platelets > 20,000/microliter after 16 days (10-74). Complete donor chimerism was demonstrated by cytogenetic and molecular methods up to day +385 post-transplant. Cyclosporin A only was used for GVHD prophylaxis. Four of 10 patients developed acute GVHD with grade I (one) and II (three) which completely resolved with treatment. Two patients died from infectious complications. Three patients died from relapse or progressive disease. Five patients are alive in remission without GVHD with a median follow-up time of 254 (93-457) days and three of five are without immunosuppression. Allogeneic transplantation of positively selected peripheral blood-derived CD34+ cells is feasible and safe and leads to long-term engraftment without severe GVHD suggesting that peripheral blood-derived CD34+ cells contain pluripotent hematopoietic stem cells. The reduced number of T cells transplanted appears to be sufficient for engraftment.