The number of circulating hematopoietic progenitor cells was determined during 47 courses of chemotherapy in 23 patients with hematopoietic malignancies. rhG-CSF was given subcutaneously to 14 patients to rescue chemotherapy-induced neutropenia following 22 courses of chemotherapy. The mean numbers of CFU-GM in patients with malignant lymphoma (ML), acute leukemia (AL) and myeloma (MM) were 56.0 +/- 58.8 (mean +/- SD), 46.7 +/- 66.0 and 11.0 +/- 11.1 CFU-GM/ml, respectively. The number of CFU-GM in MM was significantly less than in normal subjects (51.2 +/- 30.6 CFU-GM/ml). The number of CFU-GM in PB in all patients began to rise between 2 days before and 8 days after nadir of WBC count, and then reached the peak at the subsequent 5 days. The peak values of CFU-GM in ML, AL and MM were 711.3 +/- 974.7, 660.0 +/- 374.7 and 403.6 +/- 232.5 CFU-GM/ml, respectively, but there was no statistical difference among them. When ML patients were treated with rhG-CSF, the CFU-GM peak values increased as much as 5.5-folds compared with those following chemotherapy only. However, neither the period from nadir to start of increase in the CFU-GM count nor the time of the CFU-GM peak showed any significant change. These results indicate that the administration of rhG-CSF makes it possible to increase the number of circulating progenitor cells. It appears possible in most of the patients with hematopoietic malignancies to harvest the sufficient number of progenitor cells which are necessary for autologous blood stem cell transplantation.