We report on a poly(dimethylsiloxane) (PDMS) microfluidic analysis system based on mass sensitive detection. It consists of a miniaturized quartz crystal microbalance (QCM) and a flow cell made out of PDMS which is bonded to the quartz substrate. Protein A was used for the immobilization and the functionality of this undercoating was tested by accumulation of anti-protein A. The sensor system was operated in a flow-through mode and the results were compared with measurements using an injection method, where a definite amount of solution is injected into a reservoir mounted on the QCM. The conducted measurements proved the excellent performance of the new flow system. (c) 2005 Elsevier B.V. All rights reserved.