SCF enhances IL-2 mediated expansion of splenic NK cells. Four groups of C57BL/6 mice (n = 5 to 6 per group) were treated with placebo, SCF, IL-2, or a combination of SCF plus IL-2, as described in Materials and Methods. After 8 weeks, spleens were excised and weighed (A) and the absolute number of spleen cells were determined by cell enumeration with a hemocytometer (B). 1 × 10⁶ spleen cells were stained with NK1.1-PE and CD3-FITC MoAb and analyzed by flow cytometry for the NK1.1⁺CD3⁻ NK cell population. There were 2.9 ± 0.2% NK1.1⁺CD3⁻ cells in the control group, 1.8 ± 0.3% in the SCF group, 1.8 ± 0.1% in the IL-2 group, and 3.2 ± 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 splenic NK cell number was then calculated by multiplying the percentage NK1.1⁺CD3⁻ NK cells by the total spleen cell number (C). The results represent the mean weights, total splenic cell number, and absolute splenic NK cell number of each treatment group ± SEM. Statistical significance comparing the SCF plus IL-2 group to the IL-2 alone group are indicated by the asterisk, and represent P < .05 (A), P ≤ .025 (B) and P ≤ .005 (C).