A simple and highly reproducible bone marrow culture system for the generation of cytolytically active NK cells from immature precursors in the bone marrow is described. The NK cells can be generated with various sources of IL-2, including Con A-conditioned medium, supernatants from IL-2-producing cell lines and recombinant IL-2. Neither IL-1, IL-3 nor alpha/beta interferon induced significant cytotoxicity in bone marrow cells. Identification of the cytotoxic cells as NK cells was based on their phenotypic characteristics (aGM1+, Thy 1 +/-, Ly 1-2- X Ia-, RIL-2 +/-, H2+), as well as their spectrum of target specificity. The deliberate addition of peripheral blood mononuclear cells as a source of mature NK cells and elimination of cells expressing markers specific for mature NK cells indicated that the generated NK cells were descendants of precursors of NK cells harboured in the bone marrow and not derived from mature NK cells contaminating the bone marrow preparations. Thus, it was shown that not only functionally active NK cells but also their precursors are highly dependent on IL-2 for differentiation and growth. This culture system should be helpful in studying the origin of NK cells in relation to other cell lineages as well as the regulation of the maturation of NK cells from their precursors.