Inhibitory effects of extracts from peels of Citrus natsudaidai (natsumikan) encapsulated in hybrid liposomes (HL) composed of L-alpha-dimyristoylphosphatidylcholine and polyoxyethylene (20) sorbitan monolaurate on the growth of tumor cells were examined. The extracts with lower polar solvents inhibited the growth of B-16 mouse melanoma and human lung carcinoma cells, although the extracts with higher polar solvents showed no antitumor activity. In particular, the inhibitory effects of extracts with lower polar solvents encapsulated in HL were enhanced as compared with those of free extracts. Fluorescence microscopic analysis indicated that the HL including petroleum ether extracts induced apoptosis in B-16 mouse melanoma cells. On the other hand, the viability of normal human fibroblast cells was even less affected by the extracts of natsumikan. These results suggest that hydrophobic antitumor agents should be present in peels of natsumikan.