The selective induction of apoptosis of gambogic acid (GA) on MGC-803 cells and its probable molecular mechanism were studied. GA greatly inhibited (24, 48, 72 h) the growth of MGC-803 cells (by MTT); the IC₅₀ value was 0.96 μg/ml at 48 h. Meanwhile, no influence was observed on body weight, number of WBC (white blood cells) in blood or karyote in marrow of rats after GA was injected intravenously. We conclude that GA does not affect normal cells, but that it can induce apoptosis in tumor cells selectively and there were marked morphological changes. A great quantity of apoptotic cells and increasing G₂/M phase cells were observed by flow cytometry, and a significant percentage of early apoptotic cells were observed by Annexin-V/PI double staining assay. The increase of bax gene and the decrease of bc1-2 gene expressions were detected by immunohistochemistry. Activation of bax and suppression of bc1-2 may contribute to the apoptosis mechanism.