CD56⁺/CD3⁻ NK cells exhibit diminished IL-2–induced proliferation in donors receiving G-CSF. Sorted NK cells derived from unprimed blood (•, n = 6) and from G-CSF–mobilized PBPC products (○, n = 5) were incubated with IL-2 and tested for short-term (day 4) proliferation as measured by thymidine incorporation (A). There was a statistically significant decrease in proliferation from NK cells derived from PBPCs v NK cells derived from unprimed blood. NK cells were then plated in IL-2–containing media alone (no accessory cells) or in direct contact with M210-B4 feeders (B). Long-term proliferation was assessed by actual cell counts after 28 days in culture. NK cell fold expansion was determined by the number of harvested cells after culture divided by the number of CD56⁺/CD3⁻ NK cells initially inoculated into culture. NK cells derived from G-CSF–mobilized PBPC collections (PBPC NK, n = 6) gave rise to significantly fewer NK cells (P ≤ .001) than when cultures were initiated with NK cells from unprimed blood (PB NK, n = 10) irrespective of whether accessory cells were used in culture.