Peripheral blood progenitor cells (PBPC) are increasingly used as the source of stem cells in both the autologous and allogeneic settings. Based on previous early non-randomized studies reporting enhanced engraftment following fractionated autologous PBPC infusion, some centers and study groups infuse PBPC over 3 days. To study the possible benefit of multiple day PBPC infusion, 60 patients receiving high-dose chemotherapy and autologous progenitor cell transplantation (ABMT) were randomized to receive their PBPC divided over 1, 2 or 3 days. Stem cells were mobilized with G-CSF 5 microg/kg for 7 days and PBPC were collected on days 5-7. Patients received daily G-CSF 5 microg/kg i.v. over 30 min beginning 4 h after the infusion of the first aliquot of PBPCs. Toxicity was similar for the 1, 2 and 3 day infusion groups. The median time to achieve 500 neutrophils/mm3 was 10, 11 and 11 days in the groups receiving PBPCs over 1, 2 or 3 days, respectively. The median time to achieve a platelet count of 20 x 10(9)/l was 11 days for the group receiving their cells as a single infusion and 12 days in the other two groups. We conclude that expanding PBPC infusion over 2 or 3 days does not enhance engraftment or reduce toxicity.