Analytical pyrolysis coupled to capillary gas chromatogra-phy-mass spectrometry was applied to the characterization of streptococci. Eight strains of group A streptococci and six strains of group B streptococci were reproducibly differentiated by the presence of a single chemical marker which appeared only In the pyrograms of group B organisms. Pyrolysis of Isolated components of these microorganisms located the marker in the group B specific polysaccharide antigen. Extracted Ion monitoring using mass m/z 86 simplified pyrograms and permitted rapid visual identification. This approach could be used to differentiate other groups of microorganisms and indicates the potential for further correlations between the chemical structure of microorganisms and pyrolysis products. © 1987, American Chemical Society. All rights reserved.