A method for the determination of cuprous ［Cu(Ⅰ)］ in biological samples by liquid-liquid extraction-inductively coupled plasma mass spectrometry (ICP-MS) was developed. Serum and cell homogenate, cell membrane components were deproteinized with trichloroacetic acid, then the supernatant was mixed with a glycine- sodium hydroxide (NaOH) buffer (pH 12.5) to obtain a mixture with pH of 9. Then 1 000 μL of 0.05% 2,2’-biquinoline dissolved in N-pentanol was added and vortexed for 1 min. After static layering, 500 μL of the organic layer was collected in a 2 mL Teflon digestive tube, and the organic solvent was evaporated in an oven at 95 ℃. After digestion with nitric acid and hydrogen peroxide, the sample solution was subjected to inductively coupled plasma mass spectrometry and the data of cuprous content were obtained. The detection limit of this method was 0.04 μg·L-1, with relative deviation being less than 5%. The recovery was 95% to 102%. Then the method was applied to determine Cu(Ⅰ) of serum from cervical cancer patients and several types of water. The results showed that serum Cu(Ⅰ) was higher in cervical cancer patients than that in normal controls. Cuprous ions could not be detected in tap water, Nongfu Spring water, and urine. While, Cuprous ions but not divalent copper ions could be detected in cytosol and cell membrane. This method could detect trace Cu(Ⅰ) ions in the presence of 10 times concentration of Cu(Ⅱ) ion without interference.