In vitro addition of G-CSF to long-term NK cell cultures leads to decreased NK cell proliferation. Ten thousand CD34⁺/Lin⁻/DR⁻ cells from unprimed marrow were inoculated into long-term NK cell culture with G-CSF (1, 10, or 100 ng/mL) added only once at culture initiation (□) or added with each weekly half-media change (▪). A large number of NK cell progeny (NK cell fold expansion determined by actual CD56⁺/CD3⁻ NK cells emerging from culture) were derived from progenitors cultured in the absence of G-CSF after the 5-week culture. In contrast, exogenous addition of G-CSF significantly decreased NK cell expansion under all G-CSF conditions and concentrations (n = 3).