The PhellinusLinteus (PL) mushroom has been shown to possess anti-tumor properties. Through influencing lymphocytes, PL indirectly augments the host's immune system against cancer cells. PL has also been demonstrated to reduce tumor proliferation. However, the mechanisms of PL against malignant growth have not yet been fully explored. In this study, we report that PL mediates the following two activities in mouse and human lung cancer cells: cell-cycle arrest at a low concentration of PL and apoptosis in response to a high dose of PL. After exposure to a low dose of PL, G1 growth arrest occurred in the lung cancer cells. The negative growth control mediated by PL is evidenced by the decrease of the activities of cyclin-dependent kinases CDK2, 4, and 6. In contrast, at high doses, PL-induced lung cancer cells to undergo apoptosis in a dose-dependent fashion. This was evidenced by DNA fragmentation, caspase activation, and loss of clonogenecity in the lung cancer cells, all of which were lacking in the lung cancer cells treated with low concentrations of PL as well as the normal mouse lung epithelial cells exposed to either low or high concentrations of PL. The addition of the caspase inhibitor Z-VADfmk completely suppressed PL-induced apoptosis. Furthermore, the low dose of PL was able to synergize with doxorubicin to induce apoptosis in the lung cancer cells. Thus, our findings suggest that PL regulates two responses in the lung cancer cells: cell-cycle arrest and apoptosis. © 2006 Wiley-Liss, Inc.