Summary A simple in vitro technique is described for the growth of colonies from single cell suspensions of mouse bone marrow. The system involves the plating of marrow cells in agar on feeder layers of other cells, those from 8‐day‐old mouse kidney and 17th day mouse embryo being shown to be the most efficient types of feeder layers. Approximalely 400 colonies per 1 × 10 6 nucleated marrow cells were grown, using kidney cell feeder layers. A linear relationship between the number of cells plated and the number of colonies developing was demonstrated. In comparison with the marrow cells, lymph node or thymus cells did not form colonies, but a small number of colonies was formed using spleen cells. Early in the development of the colonies the dominant cell type was a large mononuclear cell with cytoplasm filled with granules staining metachromatically with toluidine blue. With growth of the colony, cells with ring or horseshoe‐shaped nuclei appeared, and a progression with further colony growth to smaller cells with segmented nuclei similar to polymorphonuclear blood cells was observed.