OBJECTIVE To demonstrate the role of cord blood serum on dte long-term culture of bone marrow and provide the experimental foundation for its clinical use, the response of the bone marrow mononuclear cells to the mixed cord blood serum was investigated on the basis of Dexter culture system. METHODS In ten samples of human bone marrow, 1 x 10(6) bone marrow mononuclear cells were planted onto the stromal cells layer and cultured for five weeks, Cobblestone area forming cells (CAFC), long-term colony-initiation cells (LTC-IC), and the nuclear cells count in the supernatant was observed. RESULTS CAFC, LTC-IC were 37.1 +/- 12.4/(ml. well), 40.9 +/- 10.6/(ml. well) respectively in control group. The accounts were increased in the group of 10% MCBS, although it was less than that in mixed cytokines group. When combined with cytokines, 10% MCBS enhanced the growth of CAFC and LTC-IC obviously compared with that when cytokines were used alone. 20% MCBS failed to substitute the role of mixed cytokines plus fatal calf serum on the expansion of CAFC, LTC-IC and NCC. CONCLUSIONS MCBS have the activity of stimulation for hematopoietic stem cells, which is analogous to SCF, IL-3, etc. Its nature and concentration remain to be investigated extensively.