Human bone marrow was cultured with alpha medium, 10(-6)M hydrocortisone and a mixture of horse and fetal calf serum in a long-term liquid culture system. An adherent layer was formed with contained fat cells, macrophages, fibroblast-like and epithelioid cells. The layer harboured myeloid cells and their presumptive precursors whilst the nonadherent cells were composed of immature myeloid elements plus mature macrophages and granulocytes whose numbers were maintained for periods of up to 12 weeks. Experiments showed that the use of serum mixtures was superior to horse or fetal calf serum alone and successful culture was accompanied by early growth of the hydrocortisone-dependent fat cells in the adherent layer.