NK cell progenitor cloning frequency decreases with exogenously added G-CSF. CD34⁺/Lin⁻/DR⁻ cells from unprimed marrow were plated in 96-well plates in limiting dilution. The absolute cloning frequency of primitive progenitors plated without G-CSF was 0.22% ± .07%. With progenitors derived from the same donor, CD34⁺/Lin⁻/DR⁻ cells were analyzed in LDA with G-CSF (10 ng/mL) added only once at culture initiation or at each weekly half-media change. G-CSF significantly decreased the cloning frequency of the starting population compared with that determined in the absence of G-CSF (n = 3 for all conditions, ^*P ≤ .001).