A high-performance liquid chromatography (HPLC) assay was developed for the quantitative determination of 2-methoxyestradiol (2ME2) in human plasma. Sample pretreatment involved a solid-phase extraction of 1 ml aliquots of plasma with C(18) micro-columns. Separation was achieved on a Novapak C(18) column (300 mm x 3.9 mm i.d.; 4 microm PS) at room temperature at an isocratic flow rate of 1 ml/min with 50% acetonitrile in water. Detection was performed at a UV wavelength of 205 nm. Calibration curves were linear in the concentration range of 1-50 ng/ml. The accuracy and precision values obtained from three different sets of quality controls analyzed in replicates of four on four separate occasions ranged from 90.7 to 105.2 and 3.17 to 8.27%, respectively.