SCF enhances IL-2 mediated expansion of BM NK cells. After eight weeks of treatment, BM was procured from two femurs in each mouse and the absolute number of BM cells were determined by cell enumeration with a hemocytometer (A). 1 × 10⁶ BM cells were then stained with NK1.1-PE and CD3-FITC and analyzed by flow cytometry for NK1.1⁺CD3⁻ NK cell populations. There were 1.3 ± 0.02% NK1.1⁺CD3⁻ cells in the control group, 1.4 ± 0.2% in the SCF group, 1.7 ± 0.1% in the IL-2 group, and 2.4 ± 0.2% in the SCF plus IL-2 group. The SCF plus IL-2 group had a significantly greater increase in percent NK1.1⁺CD3⁻ cells compared to the IL-2 alone group (P < .005). Absolute BM NK cell number was calculated by multiplying total BM cells by the percent NK cells in the BM (B). Results represent the mean absolute number from 5 to 6 mice per treatment group ± SEM. Statistical significance comparing the SCF plus IL-2 group to the IL-2 alone group is indicated by the asterisk, and represent P < .05 (A), and P ≤ .025 (B).