Here is presented a new method to detect and to classify microorganisms from their ellipsometric response in the mid-infrared range (2-12 mum) measured by FTIR ellipsometry. Apart from the ellipsometric measurements, the performance of the method also stands on the simplicity of sample preparation and data analysis. In the mid-infrared range each molecule exhibits a characteristic absorption fingerprint, thus making ellipsometry chemically selective. FIFIR ellipsometry is used here for the first time to analyze bacteria grown in culture media. Sample preparation is extremely simple and consists of the evaporation of a droplet of an aqueous suspension of microorganisms on a planar surface. Ellipsometric measurements are performed on the solid residue left on the surface after the evaporation of the droplet. Data analysis can be divided in two steps. First, simplification of the measured spectra by principal component analysis (PCA), which is a common multivariate statistical technique used to eliminate redundant information. Second, classification of the simplified spectra using a standard clustering method. As a result, we show how this method can be employed to discriminate and identify bacteria at the species level. The results of this experiment are very promising for the application of ellipsometry for analytical purposes in biochemistry and in medicine. (C) 2004 Elsevier B.V. All rights reserved.