Adherence to the culture substratum exerted a significant influence on the growth and survival of murine bone marrow cells incubated under conditions that favored the development of mononuclear phagocytes. Bone marrow cells cultured two weeks in unprocessed polystyrene dishes, refractory to cell attachment, reached a maximum or near maximum cell density by day 11. This density was stable throughout the duration of the incubation period. Characterization of the cells on day 11 revealed an essentially pure population of mononuclear phagocytes composed of round, nonadherent cells, and slightly elongated, loosely attached cells. Bone marrow cells incubated in polystyrene tissue culture dishes processed to promote cell attachment also reached a maximum cell density by day 11. However, this maximum density was only half that attained by cells incubated in unprocessed polystyrene dishes. Furthermore, continued incubation resulted in a sharp decline in cell viability and number. The cells in tissue culture dishes on day 11 represented a pure mononuclear phagocyte population composed principally of cells adherent to the surface of the dish. The subsequent analysis of cells subcultured in processed and unprocessed polystyrene dishes indicated that adherence to the substratum modulated the growth factor requirements of bone marrow-derived mononuclear phagocytes. Specifically, mononuclear phagocytes in tissue culture dishes expressed an elevated requirement for colony stimulating factor-1 in order to proliferate and survive in long-term culture.