LTC on AFT024 maintains greater levels of repopulating stem cell activity than short-term cytokine- or short-term AFT024-supported cultures. Purified fetal liver cells were cultured for 5 days in suspension with cytokines or on an AFT024 monolayer (3,000/well, 12-well tray). Additional cells from the same purification were seeded onto AFT024 monolayers (3,000/6-cm dish) and maintained in Dexter LTC for 5 weeks. At completion of both culture periods, the cells were harvested, mixed with Ly5.1 BM, and used to transplant mice. Each mouse received 20% of each culture (600 stem cell equivalents) and 4 × 10⁵ competitor BM cells (4 mice/culture). Peripheral blood cells from mice were analyzed for the presence of Ly5.2⁺ cells at 15 weeks after transplant. FL, 1.0 ± 0.57; FL/SL, 0.75 ± 0.25; FL/IL-6, 1.8 ± 0.14; SL/IL-6, 3.2 ± 0.46; FL/IL-6/SL, 1.7 ± 0.21; AFT024 5 days, 2.8 ± 0.11; AFT024 5 weeks, 32.2 ± 7.4. *P < .004, Student's t test. Error bars represent the SEM.