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The impedance technique mostly meets today's requirements of microbiological rapid methods. At relatively high prime cost for the equipmentthe advantages are marked by low personnel and material costs as wellas swiftness combined with highly flexible usage. The method is applicable for both quantitative and qualitative examinations but can fail occasionally in total count determination, especially if the sample material contains heterogeneous microbes. In model investigations with 53 strains of 17 different genera Enterobacteriaceae strains, Aeromonads and Enterococcus strains proved to be highly impedance effective. Lactobacillus strains and Pseudomonads as well as Staphylococcus aureus strains showed a low impedance effectiveness. Several strains, for example of the genera Micrococcus, Acinetobacter and Brochothrix, did notshow any changes of the medium impedance under the chosen conditions. Criterion for characterization of impedance effectiveness was the impedance detection time starting with identical initial counts (10(3) cfu/ml). Impedance effectiveness of microbes was determined at highly varying degree by the parameters of generation time, lag-phase duration and relative activity. This can lead either to wrong negative (underestimations) or wrong positive (overestimations) results of bacterial count.