Stromal fibroblasts of bone marrow, spleen and thymus origin as well as fibroblasts of peritoneal fluid origin are shown to have no difference in the ability to bind morphologically distinguishable granuloid cells in monolayer cultures. On the contrary, thymus fibroblast colonies encourage the development of myeloid cell plaques (which are formed by bone marrow hemopoietic precursor cells) a great deal stronger than do bone marrow and peritoneal fluid fibroblast colonies. It means that their different origin may be revealed when they interact with hemopoietic precursor cells but not with differentiated myeloid cells.