A simple, fast and sensitive liquid chromatography-mass spectrometry (LC-MS) method with automated on-line extraction using turbulent flow chromatography (TFC) for the determination of five catechins in human plasma was developed. In this method, after on-line extraction by its injection onto an extractor column at turbulent flow, five catechins were backwashed onto a reversed phase column via on-line column switching and separated chromatographically at a laminar flow of 1 ml min(-1). Using this tandem LC-LC-MS system, the extraction, the separation and the quantitation of five catechins in human plasma could be achieved with satisfactory selectivity and sensitivity. The limit of detection (S/N = 3) ranged from 0.6 to 2 ng ml(-1). The described procedure was very simple and rapid since no off-line sample preparation was required, total analysis time being 18.5 min.