Methylcellulose culture study revealed that the potency of recombinant granulocyte-colony stimulating factor (G-CSF) or granulocyte/macrophage (GM)-CSF to support the growth of purified target cell populations from peripheral blood of patients undergoing peripheral blood stem cell (PBSC) autotransplantation was inferior to that of PHA-stimulated lymphocyte conditioned medium (PHA-LCM) or interleukin-3 (IL-3). In liquid-suspension limiting dilution assay the cells responded directly to IL-3 by proliferation with single-hit kinetics. Hence, optimal culture for PBSC should include the use of PHA-LCM or IL-3. Further study disclosed a linear relationship between the number of CFU-GM infused into the patients and days required to achieve a granulocyte count above 0.5 x 10(9)/l.