A novel lipid compound, epolactaene, was isolated from the culture supernatant of Penicillium sp. 1689-P and it has already been reported that it induced neurite outgrowth in a human neuroblastoma cell line. In this study, we first investigated the effects of epolactaene on a human leukemia B-cell line, BALL-1 cells, and clarified that epolactaene induces apoptosis in BALL-1 cells in a dose- and time-dependent manner. Furthermore, we focused on the side chain structure of epolactaene, and chemically synthesized epolactaene derivatives. One derivative, which has a straight long alkyl chain as its side chain, induced apoptosis more effectively than epolactaene. On the other hand, other derivatives with a short alkyl side chain had weaker apoptosis-inducing actions. A good correlation was found between the apoptosis-inducing action of these compounds and their octanol/water partition coefficients (log P). These results suggested that the apoptosis-inducing activities of epolactaene and its derivatives were related to the hydrophobicity of these compounds; so that side chain structure of epolactaene is very important for its apoptosis-inducing activities. These apoptosis-inducing actions of epolactaene and its derivatives were also observed in various blood tumor cell lines and normal lymphocytes.