Stromsil precursors ran bo detected in populations of homopoietic cells by their ability to form fibroblast colonies (clones) in monolayer cultures. The stable colony-forming efficiency is reached when the initial density of explanted bone marrow or spleen cells is not less than 10 4 cells/cm 2 or when irradiated bone marrow cells are added in the capacity of a supplementary feeder. The clonogenic cells belong to slowly proliferating and extremely adhesive homopoietic tissue cells. Their radiation survival curve has a D 0 of 178 R. Fibroblast strains originating by successive passages in vitro of cells from the initial colonies retain the capacity to form stromal tissue characteristic of the corresponding hemopoietic organ; retransplantation under the kidney capsule results in the formation of bone populated with myoloid cells when the fibroblasts are transplanted from bone marrow cultures, and reticular tissue populated with lymphoid cells when the fibroblasts are transplanted from spleen cultures. It is concluded that stromal precursors are the cells responsible for the transfer of the microenvironmont typical of the given homopoiotic tissue.