In this study we assessed the in vivo antitumor activity of combined docetaxel (DOCE) and doxorubicin (DXR) treatment using 2 human breast carcinoma cell xenografts (R-27 and MX-1) in the nude mouse model. The transplanted tumors were allowed to reach exponential growth, whereupon 10 or 40 mg DOCE per kg alone (i.p.), 8 mg DXR per kg alone (i.v.), or 10 mg/kg DOCE (i.p.) and 8 mg/kg of DXR (i.v.), in the sequence of DOCE followed by DXR, were administered. The in vivo antitumor activity of combined DOCE and DXR was synergistic against R-27 and additive against MX-1. P-glycoprotein (P-gp) was detected immunohistochemically, and was highly expressed in R-27, but not in MX-1. In conclusion, DOCE may increase the antitumor activity of DXR against P-gp-positive breast cancer xenografts, such that the DOCE and DXR combination may be a useful treatment in clinical breast cancer.