Sorafenib is an orally administered multikinase inhibitor that exhibits anti-angiogenic and anti-tumor activity. Sorafenib is also known to bind to protein (>99.5%), suggesting protein binding may be involved in sorafenib pharmacokinetic variability. Albumin is a major drug-binding protein. In this study, we examined the effect of albumin on sorafenib-induced cytotoxicity using two in vitro culture cell lines, human hepatoma Huh-7 cells and androgen-independent prostate cancer PC-3 cells. The cells were cultured and incubated, and cytotoxicity was assessed. Results were confirmed by western blotting. The presence of exogenous albumin markedly blocked the sorafenib-induced cytotoxicity in the two cell lines. Albumin concentration, the change of pharmacological signal transduction as Raf-B, vascular endothelial growth factor (VEGF), and phosphorylation of MEK1/2 or ERK1/2 were found to be decreased by sorafenib. Co-incubation of warfarin, a representative coumarin anticoagulant and potent binding activity, with albumin enhanced the cytotoxic effects by sorafenib. These mechanisms depend on the high binding proper ties of sorafenib and exogenous albumin. Furthermore, we investigated the effects of endo genous albumin expression on sorafenib-induced cytotoxicity using the siRNA knockdown system or transfected expression vector assay. However, the cytotoxic effects by sorafenib showed little change either with the knockdown or overexpression of albumin. Our results suggest that particular care should be taken with albuminemia or the combined use of drugs with a high affinity for albumin, such as warfarin, and sorafenib in the treatment of cancer patients. Our findings may be useful to the cancer therapeutic strategy by sorafenib.