Electron spectroscopic imaging (ESI) is a microanalytical technique which is being used to determine elemental distributions at the resolution limit of the electron microscope. Detection and mapping of phosphorus by energy filtered imaging makes it possible to determine the organization of the nucleic acid component in nucleoprotein complexes, because phosphorus is present at much higher levels in nucleic acids than in the associating proteins. ESI has provided a method for approaching numerous questions related to chromatin structure at the level of the specific protein-DNA interactions, at the nucleosome level and at higher organizational levels of chromosome structure.