Objective: The purpose of this study is to explore RT-PCR method to set up the examination platform for detecting circulating tumor cells(CTC) in peripheral blood from metastatic breast cancer patients.The primary endpoint is to find out the correlation of existence of CTC with clinical responses and progression-free survival (PFS).Methods: The breast cancer cell line MCF-7 was serially diluted into the peripheral blood from 45 healthy donors to set up the sensitivity of RT-PCR assay.The expression of CK19 mRNA was amplified from both 49 patients and 45 healthy donors respectively.The CK19 protein quantity from plasma was measured by competitive inhibition ELISA assay.Results: The sensitivity of RT-PCR could reach 1/106-107 white blood cells with specificity of 95.6%.The objective response rate(ORR) of patients with CK19 mRNA-negative undertaken one cycle chemotherapy was significantly higher than those with positive(P