Objective To investigate the relative activity of multidrug resistance P-glycoprotein in the plasma of breast cancer patients, analyze the expression of P-glycoprotein before and after chemotherapy and evaluate its clinical value. Methods The plasma of patients and chemical reagents were mixed and distributed into the 96-well plate. The plate was read at 340 nm absorbaace before and after reaction. The variation of absorbance for each well was calculated, and then relative activity of each sample was compared to the referent compound activity. The relative activity of over 30 % was taken as positive expression of the P-gp for each sample. Results The frequency of P-glycoprotein expression was significantly increased from 16.2 % before chemotherapy to 43.2 % after chemotherapy (P<0.05). The positive expression of P-glycoprotein before and after chemotherapy were 5/6 and 9/16, respectively, in patients with positive lymph node metastasis, and 1/6 and 7/16, respectively, in patients with negative lymph node metastasis. Conclusion The results of this preliminary study suggest in P-glycoprotein expression in plasma of patients play an important role in the selection of drugs, which may be useful to evaluate the prognosis for treatment of breast cancer patients.