The notable increase of the circulating granulocyte-monocyte progenitors (PB CFU-GM) during bone marrow recovery following chemotherapy is a well known phenomenon. It has led to consider harvesting a large number of autologous stem cells by cytapheresis. Daily assessments have been conducted on the PB CFU-GM level in 9 patients with acute leukemia at the time of bone marrow regeneration after the first induction course in order to identify the circumstances of this rise. The PB CFU-GM maximum peak, of an average of 2142/ml, occurs between day 17 and day 23 after the chemotherapy has ended. A ten-fold increase of the PB CFU-GM level above normal values is maintained between 2 and 10 days. The PB CFU-GM peak coincides with that of the circulating immature myeloid cells and monocytes. The platelet rise above 100 X 10(9)/1 always occurs 2-7 days before the PB CFU-GM peak.