AbstractFor safe autografts with peripheral blood hematopoietic cells (PBSCT), better methods for determining the kinetics of stem cell populations and predicting engraftment speed after PBSCT need to be established. Current methods include culture in semi‐solid medium and measurement of CD34 cell surface antigen. In this study with only partially purified blood cells obtained from children with cancer in remission, we compared the effects of phytohemagglutinin‐stimulated lymphocyte‐conditioned medium (PHA‐LCM) and recombinant human cytokines on the growth of progenitor cells in a methylcellulose culture system. Interleukin‐3 (IL‐3) alone supported more progenitor growth than standard PHA‐LCM by a factor of 1.54 for colony‐forming unit granulocyte/macrophages (CFU‐GM) and by a factor of 1.84 for burst‐forming unit/erythroids (BFU‐E). No significant change, in terms of the number of growing colonies, was observed by adding granulocyte/macrophage colony‐stimulating factor (GM‐CSF), granulocyte‐CSF (G‐CSF), or IL‐1 to IL‐3. However, the addition of G‐CSF resulted in increased colony size. A further increase in CFU‐GM growth was observed by the addition of IFN‐γ to the combination of cytokines. No significant effect was observed when stem cell factor (SCF) was added to the combination of cytokines containing IL‐3, G‐CSF, and IFN‐γ. This analysis suggests that the combination of IL‐3, G‐CSF, and IFN‐γ may provide sufficient stimulation for the growth of human blood cells. The effects of different oxygen tensions on progenitor growth in the presence of IL‐3, G‐CSF, and IFN‐γ were also evaluated. Both CFU‐GM and BFU‐E formation were increased when the culture was grown at 5% O2, as compared with an ambient 20% O2tension. A small number of infused cells were grown in culture incorporating either IL‐3, G‐CSF, and IFN‐γ at 5% O2or PHA‐LCM at 20% O2, and the number of infused cells was correlated to the speed of hematopoietic recovery after PBSCT. Although a significant negative correlation was observed between the number of infused CFU‐GM per kilogram of the patient's body weight and the recovery of hematopoiesis under both culture conditions, a better correlation was found when the former method was applied (Plt; .001 vs.Plt; .05). These findings suggest that a culture containing IL‐3. G‐CSF, and IFN‐γ at low O2tension provides satisfactory conditions for the proliferation of blood progenitors, and that this mixture of recom