It has been shown elsewhere that infrared (IR) spectroscopy can be successfully employed for the on-line monitoring of bio-processes. A horizontal attenuated total reflection (ATR) unit connected to a portable IR-cube was used here to measure the IR absorption spectra of supernatant and microorganisms separately. The common problem of bio-film formation on the ATR was addressed before, e.g. by chemical means. We present a novel method employing the principles of ultrasonic particle manipulation to avoid and potentially remove this coating brought about by the use of fermentation broth. A novel flow cell for a horizontal ATR was developed that decreases measurement time and the undesired formation of bio-films on the ATR surface. An ultrasonic standing wave (/spl sim/2 MHz) is built up between a horizontal transducer and the ATR crystal. Yeast cells in suspension were agglomerated within certain regions by the ultrasound field and therefore settled about 3-4 times faster on the ATR when the field was switched off compared to the slow sedimentation of freely dispersed cells. After the IR spectrum had been measured, the same sound field was used to actively lift the settled material from the optical sensitive surface which therefore could be rinsed away more effectively.