CD56⁺/CD3⁻ NK cells function abnormally in donors receiving G-CSF. NK cells were selected by flow cytometry from unprimed blood (•, n = 4), from G-CSF–mobilized PBPC products (○, n = 6), and from peripheral blood of donors receiving G-CSF (▵, n = 5). There was a statistically significant difference in unstimulated cytotoxicity against K562 targets between NK cells derived from unprimed blood and NK cells derived from G-CSF–mobilized blood (^*P ≤ .05 and ^;dyP = .07) or from the PBPC product collected by lymphapheresis (P ≤ .001 at all E:T ratios). There was also a statistically significant difference at the 2 highest E:T ratios (^*P ≤ .05) between NK cells derived from G-CSF–mobilized blood and G-CSF PBPC products.