Megakaryocyte colony formation from the CXCR4-positive or -negative fraction of bone marrow cells separated by immunomagnetic bead isolation. Nonadherent light-density bone marrow cells were separated by immunomagnetic bead isolation as described in Materials and Methods and then cultured in the plasma clot system for CFU-Meg. The number of megakaryocyte colonies was determined after 12 days of culture. Results represent the mean ± SD of three separate experiments performed in triplicate. * and ** show statistical significance compared with unseparated control and are assessed asP < .05 and P < .01, respectively.