Precursor cells for NK activity, present in the light fraction of fresh mouse bone marrow, were cultivated in vitro in the presence of either CSF-1, IL-2, or a combination of both factors. In the presence of only CSF-1, strong proliferation was induced. Cells quickly passed the macrophage precursor stage and matured to typical macrophages. Neither granula formation nor NK activity were induced. Under culture conditions with only IL-2 NK activity had developed after 3 d, however, no significant proliferation occurred. In the presence of both factors strong proliferation was induced, and concomitantly, granula formation and NK activity developed. Apparently, proliferation depended on CSF-1 and granula formation, and NK cytotoxicity was induced by IL-2. When proliferating cells with strong anti-YAC-1 activity from a culture in CSF-1 plus IL-2 were further cultivated in only IL-2, the content of granula further increased, whereas proliferation gradually stopped. In contrast, when these cells from CSF-1 plus IL-2 culture were further cultivated in only CSF-1, granula disappeared and NK activity was lost, whereas sustained proliferation and differentiation to macrophages occurred. Only under culture conditions with both factors were proliferation and NK activity both maintained. More than 90% of cells from a 3-d culture in CSF-1 plus IL-2 expressed the NK 1.1. marker, whereas F4/80 was only marginally detected by FACS analysis. After two further days in culture, 70% of the cells expressed F4/80 and 60% coexpressed NK 1.1. and F4/80. By setting the size scatter in order to gate for large granular cells, a population was obtained with 100% coexpression of NK1.1. and F4/80. The data indicate that early cells of the macrophage lineage can develop into different functional and morphological directions depending on the varying influence of IL-2 and CSF-1.