Pre–CFU-s activity of the purified CCE-25–c-Kit^neg cell populations. CCE-25–c-Kit^negSca-1^pos and Sca-1^neg populations were isolated as described in “Materials and methods.” The purified cell populations were transplanted into lethally irradiated mice. After 12 days, the animals were killed. Their spleens were fixed in Tellesniczky solution to visualize spleen colonies (Primary CFU-s). Whole bone marrow from these primary recipients was transplanted into lethally irradiated secondary recipients. After 12 days, spleens from secondary recipients were fixed in Tellesniczky solution (Secondary CFU-s). These results are representative of 3 experiments.