Transplantations to restore the hematopoietic system were originally performed with cells from the bone marrow (BM) (20) which was considered the only cell source comprising repopulating progenitor cells. The discovery that chemotherapy induced the mobilization of CD34+ cells into the peripheral blood (PB) (14) gave rise to the successful autologous transplantation of PBSC (1, 13). Also cord blood (CB) was found to contain considerable numbers of "stem cells", and to date at least 42 allogeneic transplantations have been performed with this cell source (22, J. Wagner, personal communication). Further investigations led to the successful autologous transplantation of positively selected CD34+ cells from BM and PB (18), and the latest results indicate that it is promising to transplant purified CD34+ cells obtained from cytokine-stimulated donors (4, 10, 15-16). Despite such achievements it remains unclear how many "stem cells" are required per kg of the recipient and how they are phenotypically characterized. In this communication we give examples of typical differences observed by flow cytometry and clonogenic assay between the CD34+ cells contained in the different cell sources. They may explain why it is not sufficient only to analyze the CD34+ cell populations which may represent progenitors of different lineages as well as of various states of differentiation. CB CD34+ cells are early myeloid progenitor cells with the highest incidence of CFU-mix among the three cell sources. They have a high proliferative potential in vitro. They hardly coexpress B cell antigens and they are partially negative for CD38.(ABSTRACT TRUNCATED AT 250 WORDS)