Kupffer cells play an important role in keeping liver from occurrence of tumors. Apoptosis is thought to be a major mechanism responsible for the anti-tumor function of Kupffer cells. Previous studies have mainly concentrated on the direct contact and interaction between Kupffer cells and tumor cells. The present experiment is to investigate the apoptotic pathway in tumor induced by culture supernatant from activated Kupffer cells. The levels of Bax, Bcl-2 and iNOS were analyzed in an in vivo mouse tumor model, which was treated with culture supernatant from activated Kupffer cells. The results showed that the expression of Bax significantly increased while the expression of Bcl-2 decreased when tumor cells were treated with culture supernatants from activated Kupffer cells. The alteration of Bax and Bcl-2 levels resulted in an increase in the ratio of Bax to Bcl-2, which had negative correlation with the size of tumor and positive correlation with the expression of iNOS. The expression of TNF alpha and the occurrence of apoptosis were also increased in tumor treated with culture supernatants from activated Kupffer cells, compared with those which received no treatment. In conclusion, culture supernatants from activated Kupffer cells were able to change the balance between Bax and Bcl-2 in favor of the former. The ratio of Bax to Bcl-2 is a useful index to evaluate tumor apoptosis induced by Kupffer cells. Our experiment also suggests that alteration of the ratio of Bax to Bcl-2 may result from increased levels of iNOS and TNF alpha.