An ultrastructural study of diffusion chamber (DC) hematopoietic colonies derived from rat bone marrow is reported. The emphasis is placed on the structural features of macrophages and fibroblasts in cultures. The observations indicate that the two cell types are normal components of DC cultures. In older cultures both macrophages and fibroblasts contain large lipid droplets, sometimes causing difficulty in the precise characterization of the cell type. The possible origin of fibroblasts from adventitial cells if sinuses or of larger blood vessels of the bone marrow implanted in diffusion chambers is discussed. It is suggested that fibroblasts exert an enhancing influence on the growth of blood cells in DC cultures, comparable to feeder layers in in vitro cultures.