The combination of anticancer drugs used in the clinic has been based upon empiricism, and the potential permutations of currently available drugs overwhelm the clinical trials system. Recently, investigators have suggested that the combination of a blockade of vital signal transduction pathways in combination with more standard therapy might enhance anticancer effect. Using a panel of breast cancer cell lines and isobologram median effect analysis, a method of determining synergism or antagonism of drugs, we have investigated in vitro potentially clinically useful combinations of agents with the human cell lines MCF7/wt, MCF7/adr, BT474, and SK-BR-3 grown in log phase. Results were confirmed by curve shift analysis. Cells were exposed to the agent(s) for 72 h and then analyzed for cytotoxicity using a MTT (3-(4,5-dimethylthiazolyl-2)-2,5-diphenyl-tetrazolium bromide) assay. Fluvastatin, an inhibitor of prenylation with excellent tolerability in man, was chosen to disrupt signal transduction pathways and thus potentially enhance the effect of more traditional anticancer agents. Anticancer agents tested were cytotoxics used in the treatment of breast cancer, trastuzumab, and rapamycin as an inhibitor of the AKT pathway. Fluvastatin combined with trastuzumab demonstrates global synergy of cytotoxic effect that is confirmed by apoptosis assay. These effects could only be partially reversed by adding farnesol or geranylgeraniol to restore prenylation. Epirubicin is also synergistic with fluvastatin in three of the four cell lines. Rapamycin, an inhibitor of MTOR, was synergistic with fluvastatin in two of the four cell lines and antagonistic in two other cell lines. The combination of fluvastatin or another inhibitor of prenylation and trastuzumab may be attractive for clinical development as the effect of trastuzumab in Her2/neu positive breast tumors is incomplete as a single agent.