Objective: To analyze the regulation effect of MDR-1 gene inhuman breast cancer cell by the differentiating agents, sodium butyrate and dimethyl sulfoxide. Methods: 1. A sensitive assay, RT-PCR, was used to measure the mRNA level before and after the treatment of sodium butyrate, DMSO, using b -actin as control; 2. Evaluated the effect of sodium butyrate, DMSO on MDR-1 gene expression of human breast cancer at the protein level by immunoflow cytometry; 3. P-glycoprotein function was examined after accumulation of the fluorescent drug, Phodamine-123, by flow cytometry; 4. Chemosensitivity to doxorubicin was analyzed using the MTT assay. Results: Sodium butyrate and DMSO were found to increase the MDR characteristics on MDR-1 gene, MDR-1 expression levels, P-glycoprotein function and chemosensitivity to doxorubicin. Conclusion: sodium butyrate, DMSO can modulate the MDR-1 gene at gene level, protein level, protein function level and cell level.