Objective     To observe the synergistic effects of extracellular signal regulated kinase (ERK) inhibitor PD98059 and bufalin on the apoptosis of K562 cells.      Methods     The percentage of viable cells was determined by trypan blue exclusion. Cell apoptosis was examined by morphology and flow cytometry

and the bcl-2 protein was measured by western blot analysis.      Results     ERK inhibitor alone inhibited the proliferation of cells

downregulated bcl-2 protein and induced apoptosis of K562 cells; bufalin was proved to have synergistic effect on the PD98059-induced apoptosis

and downregulated bcl-2 protein markedly.      Conclusion     PD98059 has synergistic effect with bufalin to induce the apoptosis of K562 cells

probably by the mechanism of bcl-2 protein downregulation.