By using anti-Nk-1 antiserum, we detected a significant proportion of Nk-1+ cells in bone marrow (BM) with low lytic activity that can be slightly enhanced with interferon (IF). These BM Nk-1+ cells also bind to YAC-1 cells. Because Nk-1 antigen has been found to mark NK cells, BM appears to harbor immature precursors to natural killer (NK) cells. We therefore used concanavalin A (Con A)-conditioned medium to culture BM cells to induce differentiation of the putative NK precursors. After 3 to 4 days in culture, cytotoxic activity to YAC, that peaked at 6 to 7 days, was consistently generated, and the activity still could be detected after 10 to 14 days in culture. In contrast, experiments using spleen cultures, performed in a similar manner, showed peak activity after 4 to 5 days and the activity declined thereafter. The cytotoxic activity of cultured BM cells was also higher than that of cultured spleen cells. Cultures of BM cells from old mice have good cytotoxic activity. The cytotoxic cells generated were Nk-1+ and Qa-5+. Furthermore, these culture conditions did not maintain the proliferation of CFU-C cells.