We have determined the effect of delayed addition of G-CSF after chemotherapy on PBPC mobilization in a group of 30 patients with high risk breast cancer (HRBC) undergoing standard chemotherapy followed by high-dose chemotherapy (HDCT) and autologous SCT. Patients received FAC chemotherapy every 21 days followed by G-CSF at doses of 5 microg/kg/day starting on day +15 (groups 1 and 2) or +8 (group 3) after chemotherapy. PBPC collections were performed daily starting after 4 doses of G-CSF and continued until more than 2.5 x 10(6) CD34+ cells had been collected. In group 1, steady-state BM progenitors were also harvested and used for SCT. Groups 2 and 3 received PBPC only. The median number of collections was three in each group. Significantly more PB CD34+ cells were collected in patients receiving G-CSF starting on day 8 vs day 15 (9.43 x 10(6)/kg and 6.2 x 10(6)/kg, respectively) (P < 0.05). After conditioning chemotherapy all harvested cells including BM and PBPC were reinfused. Neutrophil and platelet engraftment was significantly faster in patients transplanted with day 8 G-CSF-mobilized PBPC (P < 0.05) and was associated with lower transplant related morbidity as reflected by days of fever, antibiotics or hospitalization (P < 0.05). Both schedules of mobilization provided successful long-term engraftment with 1 year post-transplant counts above 80% of pretransplant values. In conclusion, we demonstrate that delayed addition of G-CSF results in successful mobilization and collection of PBPC with significant advantage of day 8 G-CSF vs day 15. PBPC collections can be scheduled on a fixed day instead of being guided by the PB counts which provides a practical advantage. Transplantation of such progenitors results in rapid short-term and long-term trilineage engraftment.