SCF enhances IL-2–mediated expansion of NK cells in the blood. After eight weeks of treatment absolute leukocyte number per mL in blood was obtained (A) and then multiplied by the percent NK1.1⁺CD3⁻ NK cells determined by flow cytometry to yield the absolute number of NK cells per mL of blood (B). There were 4.8 ± 0.8% NK1.1⁺CD3⁻ cells in the control group, 4.5 ± 0.3 in the SCF group, 10.3 ± 2.8% in the IL-2 group, and 9.8 ± 1.1% in the SCF plus IL-2 group. Results represent the mean absolute number from 5 to 6 mice in 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 and B).