Abstract Using methods described, long-term tissue cultures of human bone marrow pass through three typical growth phases : (1) a myeloid phase during which myeloid cells can be recognized; (2) a round-cell phase during which histioid and monocytoid cells predominate; and (3) a fibroblastic phase which can be maintained thereafter. The three phases are of variable onset and duration. In a culture from a patient with carcinoma of the lung, isolated colonies of polygonal cells appeared on the 51st day of continuous cultivation. These were removed and subcultured. By means of trypsinization, the sheets of polygonal cells were transferred and subcultured in bulk and in a manner making it possible to obtain replicate cultures. The morphologic characteristics and growth behavior of the strain of cells (Detroit-6 strain) are similar to those observed for HeLa cells. The pattern of growth of the Detroit-6 strain is that of epithelial cells, presumably of metastatic origin from a carcinoma of the lung. The susceptibilities of the Detroit-6 strain to infection with various viruses (poliomyelitis, Coxsackie, herpes simplex, and influenza) are similar to those observed in the HeLa strain of human malignant epithelial cells originating from a carcinoma of the cervix. The possible values of long-term tissue culture of human bone marrow have been discussed.