To explore the DNA binding proteins of mouse mbcl-2 regulatory region in the apoptotic process of mouse fibroblast cell line (C3H 10 T1/2 Cl 8) and its transformed counter part, the mouse bcl-2 (mbcl-2) regulatory region was amplified by PCR. The PCR products were subcloned into pGEM-T vector system and then checked by sequencing. Using the product as probe, DNA binding proteins of mbcl-2′s regulatory region in nuclear protein extracts of both cell lines were studied. Southwestern blotting results showed that a 53 ku protein binds with the probe.Its binding signal strengthened after exposing both cells to 5-Fu for 12 h, as another DNA binding protein (100 ku) also binds with the probe though its binding signal weakened after 5-Fu treatment. The results suggested that p53 protein may be the negative regulatory factor of mbcl-2 gene. An unidentified 100 ku protein may be the positive regulatory factor of mbcl-2 gene.