Hematopoietic The International Journal of Cell Cloning express the CD34 antigen on their surface. Their proportion in a harvest can provide a quick and reliable measure of the hemopoietic efficiency of the harvest. In ANLL a variable proportion of blast cells express the same antigen: the aim of our study was to investigate whether clonogenic ANLL blast cells are CD34+. Seven peripheral blood samples of patients with ANLL were studied. From these, CD34+ cells were isolated using My10 antibody, immunomagnetic beads and chymopapain. About 60% of CD34+ cells were recovered in the positive fraction, while the negative fraction contained less than 1% CD34+ cells. Cloning these fractions resulted in an almost complete absence of CFU in the negative fraction. This study shows that leukemia-colony forming cells are, as normal CFU, CD34+ and that they represent only a small fraction (less than 1%) of the CD34+ leukemic population.