NK cell progeny of primitive progenitors cultured with G-CSF function abnormally. CD34⁺/Lin⁻/DR⁻ cells from unprimed marrow were cultured in long-term NK cell culture with (○) or without (•) 1 ng/mL G-CSF added only once at culture initiation. Week 5 progeny of these cultures were tested in cytotoxicity assays against K562 targets (n = 3). The E:T ratio was offset to represent the absolute NK cell number present in each population (95% NK cells without G-CSF v 64% with G-CSF), and statistics are reported using the higher absolute NK cell E:T ratio for G-CSF–derived progeny compared with the lower E:T ratio for control NK cell progeny grown without G-CSF. Differentiated NK cells grown in the absence of G-CSF exhibited potent lytic function against K562 targets, which was significantly diminished when G-CSF was added at culture initiation (^*P ≤ .02).