Standard bacterial tests such as growth inhibition tests are the only convenient investigation method to detect the impacts of water soluble drugs, e.g. antibiotics, on bacteria. State of the art methods for determining the inhibiting effect of toxic components are cumbersome, because they are for parallelization not feasible. The principle of these inhibition tests is the detection of cell numbers by measuring the optical density. With this time-consuming and complicate method only endpoint detection and no monitoring of cell growth is possible, which is obviously a drawback. Furthermore, colored and turbid components cannot be tested. A new microsystem is introduced in this paper overcoming the mention problems. A continuously working conductivity sensor system for high throughput application is developed. With this miniaturized system a reduction of sample volume from 50 ml down to 12 μl becomes possible. An additional advantage is the possibility of online monitoring during biological tests and cell growth detection in high turbid or colored samples, which could not be investigated before. The sensor system is calibrated for growth inhibition of Pseudomonas putida, tested with antibiotics. © 2006 Elsevier B.V. All rights reserved.