PURPOSE Recently, high-dose chemotherapy with peripheral blood stem cell (PBSC) rescue has been developed for poor risk testicular germ cell cancer. In this study, we investigated the optimum timing for harvesting PBSCs with the use of bleomycin + etoposide + cisplatin (BEP) chemotherapy, which is a well known first-line regimen for the testicular cancer. MATERIAL AND METHOD Peripheral blood CD34-positive cell ratios were measured during a total of 10 courses of BEP chemotherapy in 6 patients with metastatic germ cell cancer between 1996 and 1998. We performed 4 apheresis in 3 patients during this period. Recombinant human granulocyte-colony stimulating factor (rhG-CSF) was administrated from the day on which the neutrophil count decreased less than 1,000/microliter. RESULTS The peripheral blood CD34-positive cell ratios became maximum (3.0-24.6%; average 10.0%) on the day 18 to 21 (median day 19) of BEP chemotherapy with rhG-CSF administration. The maximum ratios of peripheral blood CD34 positive cells were achieved when the number of leukocyte were 6,880-23,600/microliter and exceeded 6,000/microliter after the 18th day of BEP chemotherapy. The average number of collected CD34 positive cells was 9.5 x 10(6)/kg at a single apheresis, and 12.6 x 10(6)/kg per patient. CONCLUSION Efficient hematopoietic progenitor cells were mobilized by BEP chemotherapy with rhG-CSF administration of first-line setting. Our results suggest that the optimum timing of PBSCs harvest is the day when the numbers of leukocyte exceed 6,000/microliter after the 18th day of BEP chemotherapy and the following day.