Human blood mononuclear cells were prepared from peripheral blood or single donor plateletpheresis residues by depletion of adherent cells and T lymphocytes. In double-layer soft agar culture, 5 x 10(5) such cells yielded from 33 to 165 colonies. For liquid culture, these cells were suspended in McCoy's 5A medium with 15% fetal bovine serum and 0%, 20%, or 40% conditioned medium (CM) and incubated for up to 14 days at 37 degrees C in a humidified 5% CO2–95% air atmosphere. The number of cells in cultures with CM decreased about 0%-10%, while cell counts from cultures without CM decreased about 45%-65%. In cultures with CM, 5%-20% of the cells were classified as blasts after 3–5 days. After 7- 11 days, blasts and promyelocytes comprised up to 53% of all cells. After 9–11 days, cells with specific granules and maturing nuclei comprised up to 56% of all cells. At 11 days, up to 66% of the cells contained peroxidase-positive granules. Cultures without CM contained no more than 5% blasts and promyelocytes and less than 5% maturing granulocytic cells. 3H-thymidine and Na235SO4 incorporation reached a peak at 3–5 days and at 5–11 days, respectively, in cells from cultures containing CM.