Marrow or Peripheral Blood for Hematopoietic Engraftment

To the Editor: Implicit in the suggestion of Storb et al.¹ that normal human peripheral blood may be used as a supplementary source of cells for hematopoietic engraftment is the belief that true hematopoietic stem cells circulate. This is a view with which we concur,² and it has the support of circumstantial evidence derived from clinical experience.^{3 4 5} In this regard it should be noted that the demonstrable presence, in human blood, of progenitor cells committed to differentiation along established pathways^{6 7 8 9 10} is irrelevant to the goal of successful engraftment, since these cells have a limited capacity for self-replication.¹¹ Using an.