Bax gene therapy for human osteosarcoma using cationic liposomes in vivo. The purpose of this study was to evaluate the anti-tumor effect of human osteosarcoma (HOSM-1) tumor xenografts in nude mice via transfer of the Bax gene using cationic liposomes. The HOSM-1 tumors transplanted into nude mice grew to 5-6 mm in diameter. Following growth of the tumor to this size, liposomes with the Bax plasmid were applied locally to the peripheral tumor (day 0) and were applied 3 times per week for 2 weeks (6 times in total). The tumor growth inhibitory effect was evaluated by measuring the tumor volume up to day 40. The expression of Bax was observed by immunohistochemical analysis and apoptosis was detected using the TUNEL assay. Tumor growth increased only slightly during the administration period, and tumor volume on day 50 was 43% of that in the saline control group. In the tumor margin 48 h after the completion of administration, Bax immunoreactivity was detected and apoptotic cells were clearly increased. Since these results suggested that Bax gene therapy using cationic liposome induced apoptosis in HOSM-1 tumor in vivo, we anticipate that this therapy will be useful for the treatment of osteosarcoma.