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Summary. Haemopoietic reconstitution (HR) using autologous peripheral blood stem cells (PBSC) was attempted after intensive chemotherapy or chemoradiotherapy in two patients with relapsed acute non-lymphoblastic leukaemia (ANLL). The PBSC were collected by leukapheresis very early in first remission and cryopreserved in liquid nitrogen. Both patients demonstrated early evidence of trilineage engraftment. The first patient received melphalan 200 mg/m2 followed by rescue with 1·3 × 108 mononuclear cells/kg body weight containing 29 × 104 granulocytemacrophage progenitor cells (CFU-GM)/kg, and HR was evident by Day 14. The second patient was treated with supralethal chemoradiotherapy followed by rescue with 3·0 × 108 mononuclear cells/kg containing 23 × 104 CFU-GM/kg. He demonstrated early engraftment with near normal peripheral blood counts by Day 16. There was a subsequent fall in both bone marrow cellularity and peripheral blood counts to a level of low but persistent activity. There was a further phase of haematological recovery from 8 weeks following transplantation with an increase in peripheral blood counts and bone marrow cellularity until final relapse at 13 weeks. This study demonstrates that circulating stem cells have haemopoietic reconstitutive capacity, previously only shown with buffy coat cells from chronic granulocytic leukaemia. The minimum number of PBSC required for satisfactory engraftment remains unknown, although it seems probable that the ratio of pluripotent stem cells to committed progenitor cells is lower in very early remission peripheral blood than in either allogeneic normal bone marrow or autologous bone marrow collected laterin stable remission. The question of leukaemic contamination of the PBSC remains to be answered.