Bacterial tests such as growth inhibition tests are the only convenient investigation method to detect the impacts of toxic water soluble materials, e.g. antibiotics, on bacteria. The principle of these inhibition tests is the detection of cell numbers by measuring the optical density. With this time consuming cumbersome and complicate method only endpoint detection and no monitoring of cell growth is possible. In this paper we present a conductivity measurement system based on the theory of Maxwell-Fricke. The intention of this work was to reduce the sample volume and have a standard screening platform, therefore we choose the 1536 microtiter plate format without a bottom foil as screening system for the growth inhibition test. The equivalent circuit models of the electrode-electrolyte interface and their influence to biological species were discussed. The presented sensor system in this paper can distinguish living and death cells during the continuous biomass monitoring. © 2006 Elsevier B.V. All rights reserved.