³H Thymidine labeling of 5-FU cells. Bone marrow cells selected in 5-FU, IL-3, and SCF were incubated for 48 hours on monolayers of retroviral producer cells and labeled for 24 hours in the presence of ³H thymidine. (Top panels) Cells incubated with producer line 1MI-▵SCF. (Bottom panels) Cells incubated with producer line 1MI and exogenous soluble SCF (100 ng/mL). (Left-hand panels) Photographed at 400× original magnification; (right-hand panels) 1,000× (oil immersion). Arrowheads indicate examples of labeled cells.