Alteration and extraction of lipids occur during the preparation of biological tissues for electron microscopy. This problem has rendered the interpretation of membrane fine structure in molecular terms particularly difficult. By using a water-miscible, glutaraldehyde-carbohydrazide (GACH) resin polymerized at neutral pH, we have previously demonstrated lipid retention in biological tissues during embedding. Since A. laidlawii membranes have been characterized with regard to the composition and structural organization of the lipid component, they provided an ideal model system for direct visualization of the retained lipids after GACH embedding.High resolution micrographs of the membranes were taken on a Hitachi HU-11B electron microscope. At the same time, the instrument was calibrated according to established guidelines and found to give a standard deviation of 2.9% in magnification over a period of approximately one year. A. laidlawii strain B cells were generously provided by Dr. James Gilliam. The presence of an intact, hydrophobic core in sections of GACH-embedded membranes was confirmed by vanatomolybdate staining of the thin sections.