Malignant cystosarcoma phyllodes (MCSP) is a rare breast tumor. Chemotherapeutic regimens for treatment of MCSP have not been established. We previously established an MCSP xenograft line MC-3-JCK. In this study, we established a new MCSP xenograft line, MC-10-JCK, by serial transplantation in nude mice. We studied the chemosensitivity of these two MCSP tumor xenografts to anticancer drugs in vivo. We also examined the expression of multidrug resistance-related proteins such as P-glycoprotein (Pgp) and multidrug resistance-associated protein (MRP) by immuno-histochemical analysis. These two xenografts were sensitive to doxorubicin, vincristine and cyclophosphamide in vivo. Immunohistochemically, clinical specimens and xenografts were negative for Pgp and MRP expression. These results are consistent with the chemosensitivity of human MCSP to lipophilic anticancer compounds.