We have adapted the stroma-dependent long-term bone marrow culture (LTBMC) system to study the development of human natural killer cells (NK) from the CD34+/HLA-DR- (CD34+/DR-) BM mononuclear cell (BMMNC) population. The CD34+/DR- population does not express any known antigens associated with myeloid or lymphoid lineage and has been shown by us and others to contain primitive hematopoietic progenitors capable of both self-renewal and differentiation to myeloid lineage. CD34+/DR- cells obtained from normal human BM by fluorescence-activated cell sorting were plated on allogeneic, irradiated BM stromal layers. After 5 weeks of culture in the presence of media containing recombinant interleukin-2 and human serum, 147- ± 21-fold expansion of cells with the morphologic appearance of large granular lymphocytes was observed. Cultured cells (84.8% ± 1.5%) expressed the characteristic CD56+/CD3- phenotype of NK. A proportion of CD56+/CD3- cells expressed other markers of lymphoid lineage that have been associated with mature NK, including CD2 (7.8% ± 1.2%), CD7 (19.5% ± 2.8), CD8 (3.1% ± 1.0%), and CD16 (4.5% ± 1.3%). The cultured cells did not express other antigens associated with T-lymphocyte (CD3, CD5, T-cell receptor [TCR] α/β and TCR γ/δ), B-lymphocyte (CD19), myeloid (MY8, CD33, and CD71), or monocytoid (CD14 and CD15) lineage and did not express the CD34 antigen associated with hematopoietic progenitors present on the starting population. This NK population was cytotoxic against both K562 (E:T 20:1; 79% ± 1.9%) and Raji (E:T 20:1; 38% ± 5.7%) target cell lines. The NK progenitor frequency in the CD34+/DR- cell population determined by limiting dilution of CD34+/DR- on stromal layers followed by a functional chromium release assay against K562 targets was 1:169 ± 50 CD34+/DR- cells. The data suggest that human LTBMC developed to study myeloid differentiation can be modified to study the origin and development of the NK and possibly other lymphoid lineages. Modified cultures show that cells with morphologic, phenotypic, and functional characteristics of NK can be derived from a population of BMMNC with the phenotype of primitive hematopoietic progenitors and without phenotypic evidence of lymphoid- or myeloid-lineage commitment. Further studies will address the cell of origin and the ontogeny of human NK and other lymphoid lineages.