In a liquid culture of human bone marrow, the development of fibroblast colonies begins to take place on day 4. Twenty per cent fetal calf serum is used as the stimulus for fibroblast colony growth. Human bone marrow contains 47 +/- 4 fibroblasts colonies per 2 X 10(5) nucleated cells plated. Bone marrow fibroblast cultures using agar or methylcellulose restrict colony formation. This fibroblast culture method may be useful for pathognomonic and clinical studies in hematological disorders.