Nucleoids isolated from Escherichia coli at low salt concentrations in the presence of spermidine (Kornberg et al., Proc. Natl. Acad. Sci. USA, 71, 3189-3193 (1974)) retain large amounts of protein and RNA and are, thus, potentially useful in structural and other studies, However, these preparations have neither been visualized nor extensively characterized with regard to their protein and other components. We have investigated this type of nucleoid preparation and here supply both Light and electron microscope appearances and a description of the DNA-associated proteins. Light microscopy is used to follow the stages of nucleoid release and to demonstrate characteristically rounded nucleoids after chloramphenicol treatment of the cells from which the nucleoids were isolated. The nucleoids are ''envelope-associated'' particles. Electron microscopy shows an irregular central core that is partially covered with small, membranous vesicles. A significant fraction of the nucleoids have a characteristic doublet/dumbbell-shaped appearance by light microscopy, The nucleoids contain large amounts of protein and RNA in addition to DNA, The DNA and RNA are rendered acid-soluble by very low levels of nucleases, indicating an open structure. A small group of proteins, including H-NS, FIS, HU, and RNA polymerase, is released from the particles upon enzymatic digestion of the DNA. (C) 1997 Academic Press.