Natural killer (NK) cell differentiation from pluripotent CD34(+) human hematopoietic stem cells or oligopotent lymphoid progenitors has already been reported. In the present study, long-term cultures of the CD56(-)/CD34(-) myeloid-like adherent cell fraction (ACF) from umbilical cord blood (UCB), characterized by the expression of CD14(+) as well as other myeloid markers, were set up with flt3 ligand (FL) and interleukin-15 (IL-15). The UCB/ACF gradually expressed the CD56 marker, which reached fairly high levels (approximately 90% of the cells were CD56(+)) by day 15. FL plus IL-15-driven ACF/CD56(+) cells progressively expressed a mature NK functional program lysing both NK- and lymphokine-activate killer (LAK)-sensitive tumor targets and producing high levels of interferon-gamma (IFN-gamma), granulocyte-macrophage colony-stimulating factor, tumor necrosis factor alpha, and IL-10 upon stimulation with IL-12 and IL-18. Similar results were obtained when highly purified CD14(+) cells from UCB were cultured with FL and IL-15. In contrast, UCB/CD34(+) cells cultured under the same conditions showed a delayed expression of CD56 and behaved functionally differently in that they exhibited NK but not LAK cytotoxicity and produced significantly fewer cytokines. Kinetic studies on the phenotype of UCB/ACF or UCB/CD14(+) cells cultured in the presence of FL and IL-15 showed a rapid decrease in CD14 expression after day 5, which reached levels of zero by day 20. Approximately 60% of the CD56(+) derived from the UCB/ACF or the UCB/CD14(+) cells coexpressed CD14 by day 5. Taken together, our data support the role of CD14(+) myeloid-like cells within UCB as a novel progenitor for lymphoid NK cells.