Limiting dilution transplantation of leukemic cells reveals the frequency and cell surface expression of the leukemic initiating population. (Ai) Sorting of populations from bulk bone marrow in a leukemic donor mouse. Bulk bone marrow was sorted into 3 populations (B220⁺Mac-1⁺/Gr-1⁺, B220⁺Mac-1⁻/Gr-1⁻, and B220⁻Mac-1⁺/Gr-1⁺) and transplanted to lethally irradiated recipients. (Aii-iv) Peripheral blood from transplant recipients evaluated for the CD45.2/CD45.1 ratio at 1 month after transplantation. (Av) Development of leukemia in the bone marrow of mice receiving the B220⁺Mac-1⁻/Gr-1⁻ population. (B) Limiting dilution transplantation was used to determine the frequency of the LIC in bulk bone marrow.