Ganoderma lucidum, a medicinal mushroom, has been known to have antimetastatic and anti-carcinogenic bioactivities and is widely used in Asian countries in complementary and alternative medicine. However, there is no information regarding its combined usage with tamoxifen and doxorubicin in breast cancer treatment. We investigated the interactions between G. lucidum and tamoxifen or doxorubicin in an MCF-7 human estrogen receptor-positive breast cancer cell line. The antiproliferative properties of 6 extracts were assessed using the WST-8 method. The most effective extract in inhibiting MCF-7 cell viability was then evaluated in terms of its antimetastatic activity by the Boyden chamber assay. Apoptosis and cell cycle assays were performed by flow cytometry. G. lucidum ether extract (G. Ether) was the most effective extract in inhibiting cell viability, among other extracts, with half-maximal inhibitory concentrations of 100 and 12.82 mu g/mL at 48 and 72 hours, respectively. We found that G. Ether is capable of inducing apoptosis and changing cell cycle dynamics. However, incubation with G. Ether did not significantly affect MCF-7 cell motility. We then assessed the interactions between G. Ether and tamoxifen or doxorubicin in MCF-7 cells. The interactions between G. Ether and cancer therapeutics were examined by combination index analysis and MacSynergy II software. Interestingly, G. Ether increased the antiproliferative effect of tamoxifen but exhibited strong antagonism with doxorubicin in the MCF-7 cell line.