Recently eukaryotic chromosomes have been shown to consist of a repeating subunit, called the nucleosome. Although electron microscopy, neutron scattering and X-ray diffraction have been used to determine the low resolution structure of the nucleosome, these techniques have yielded little information about the disposition of nucleosomes within chromosomes. Electron microscopy has produced many models for chromosome structure based on uniform fibres of 150-500A diameter or on globular 'superbeads. Unfortunately the models are based on microscope images that fail to reveal the strong structural periodicities shown by X-ray scattering to be characteristic of isolated chromatin in solution. Moreover it has not been demonstrated that the chromosomes of living cells are composed of such fibres. We have used low-angle X-ray scattering to investigate the organization of chromosomes in vivo and to account for the previously observed inconsistencies in many X-ray and electron microscope observations. We report here that chicken erythrocytes have a 400 A periodicity due to a nuclear structure that is directly related to the 300 A side-by-side packing of chromosome fibres revealed by electron microscopy of embedded cells, and that this periodicity can be preserved in isolated nuclei provided that the proper buffers are used.