In a previous study, we speculated that the early phase of hematopoietic recovery after PBSC transplantation (PBSCT) is rapid because of the increased production of endogenous cytokines by co-transfused monocytes and lymphocytes (Kawano Y, et al. Blood 81:856, 1993). To clarify this point, the serum level of G-CSF was measured using an ELISA, and various other cytokines, including GM-CSF, macrophage-CSF (M-CSF), SCF, IL-6, IFN-gamma, and soluble IL-2 receptor (IL-2R), were tested for comparison in children receiving conventional or high-dose chemotherapy and autologous transplantation with unmanipulated or purified PBSC. Serum G-CSF levels in patients receiving conventional chemotherapy (n = 21) or PBSCT without exogenous G-CSF treatment (n = 19) increased to 1245 +/- 2337 pg/ml and 2741 +/- 2331 pg/ml, respectively. Likewise, the peak level of G-CSF in patients who did not receive G-CSF was statistically equivalent to the trough level in those who did. There was no significant difference in the speed of hematopoietic recovery with or without G-CSF treatment in both the conventional chemotherapy and PBSCT cohorts. In addition, no meaningful change was observed in the kinetics of other tested factors in either conventional therapy or PBSCT settings, regardless of whether the patient did or did not receive G-CSF. Endogenously produced serum peak G-CSF levels after PBSCT with purified CD34+ cells were identical to those after the same procedure with unmanipulated cells. These results confirm that children receiving intense chemotherapy followed by autologous PBSCT produce a high level of G-CSF during the cytopenic period that is not due to the infusion of a large amount of facilitating cells capable of producing G-CSF.