Primitive progenitors from G-CSF–mobilized PBPC collections give rise to fewer NK cell progeny. Ten thousand primitive progenitors from unprimed marrow (CD34⁺/Lin⁻/DR⁻) or from G-CSF–mobilized PBPC products (CD34⁺/Lin⁻/DR⁻ and CD34⁺/Lin⁻/DR⁺) were plated on pre-established allogeneic irradiated stroma for long-term NK cell culture. NK cell fold expansion was reported as the number of total cells harvested after 5 weeks. All cultured populations contained < 90% CD56⁺/CD3⁻ NK cells. CD34⁺/Lin⁻/DR⁻ cells from unprimed marrow gave rise to more NK cells than either population selected from G-CSF–mobilized PBPCs.