The antitumor activities of ethaselen （BBSKE） in combination with cisplatin （CDDP） in vitro have been investigated in human stomach cancer cell line BGC 823 and human lung cancer cell line PG-BE1. MTT method was used to assess the individual effects of ethaselen and cisplatin and their combined effects on cell proliferation of BGC 823 cells and PG-BE1 cells. Additionally, we used the classic median effect theory to calculate the combination index （CI） of ethaselen and cisplatin with different dose regimes in combination, and compared the effective dosages of cisplatin in individual treatment and combination treatment. When ethaselen and cisplatin were used in combination, a synergistic effect was observed. The most remarkable synergistic effect was observed when the dose ratio of cisplatin to ethaselen was 2：3 in BGC 823 cell line and 1：3 in PG-BE1 cell line. The dose of cisplatin could be decreased markedly in combination group to reach the same inhibitory effect, and this effect was gradually raised with the increase of the concentration of drugs.
