The existence of primitive hematopoietic progenitors in mobilized peripheral blood is suggested by clinical, phenotypic and in vitro cell culture evidences. In order to quantify primitive progenitors, 32 leukaphereses from 15 patients with lymphoid malignancies were investigated for the growth of multilineage colony-forming units (CFU-Mix), erythroid burst-forming units (BFU-E) and granulocyte-macrophage colony-forming units (CFU-GM) in the absence or presence of recombinant stem cell factor (SCF), a cytokine which selectively controls stem cell self-renewal, proliferation and differentiation. Primitive progenitors were also quantitated by means of a long-term assay which allows the growth of cells capable of initiating and sustaining hematopoiesis in long-term culture (LTC-IC). Addition of SCF (50 ng/ml) to methyl-cellulose cultures stimulated with maximal concentrations of G-CSF, GM-CSF, interleukin 3 and erythropoietin significantly increased the growth (mean +/- SE) of CFU-Mix (7.7 +/- 1.7 versus 2.4 +/- 0.6, p < or = 0.0001), BFU-E (47 +/- 10 versus 32 +/- 6, p < or = 0.002) and CFU-GM (173 +/- 31 versus 112 +/- 20, p < or = 0.0001). Mean (+/- SE) percentages of SCF-dependent CFU-Mix, BFU-E and CFU-GM were 60 +/- 5%, 19 +/- 5%, and 33 +/- 4%, respectively. Mean (+/- SE) LTC-IC growth per 2 x 10(6) nucleated cells was 221 +/- 53 (range, 2 to 704). Linear regression analysis demonstrated a statistically significant correlation (r = .87; p < or = 0.0001) between LTC-IC and SCF-dependent progenitors. In conclusion, our data suggest that: A) the optimal quantification of mobilized progenitors requires supplementation of methylcellulose cultures with SCF, and B) in vitro detection of SCF-dependent progenitors might represent a reliable and technically simple method to assess the primitive progenitor cell content of blood cell autografts. Such in vitro evaluation of immature hematopoietic progenitors might be clinically relevant for predicting the reconstituting potential of autografts.