Circulating stem cells (CSC) are well documented in animals and humans. Though their function in normal conditions remains obscure, autologous CSC seem capable of restoring hemopoiesis after myeloablative treatment. With cell separators CSC may be harvested in adequate number, and collection may be further improved giving chemotherapy and/or GM-CSF that mobilize stem cells into the circulation. Due to the high number of progenitor cells infused, hematologic reconstitution is more rapid with CSC than with marrow cells. Autologous blood stem cell transplantation (ABSCT) is increasingly employed in a variety of hematologic malignancies and in some solid tumors. CSC allow transplantation in patients previously irradiated on the sites of harvest or with marrow tumor involvement, and probably decrease the risk of infection by shortening the duration of post-graft aplasia. Their use is also encouraged by a belief that, along with CSC, a large number of immunocompetent cells are infused that may exert an anti-tumor effect. A lower tumor contamination of CSC as compared to marrow is an attractive matter, but remains to be demonstrated. Standardization of cell cloning assays, identification of monoclonal antibodies to recognize the surface antigens expressed on progenitor cells, and definition of advantages of ABSCT are items of future work.