A new, water-miscible polymer, glutaraldehyde-carbohydrazide (GACH), has been developed for electron microscopy. Its parameters and use are described. Fine structural detail was observed in uranyl acetate and lead citrate-stained thin sections of glutaraldehyde-fixed GACH-embedded tissues. Cells of rat liver, duodenum, epididymis, and kidney and erythrocyte ghosts appeared similar to those in routinely fixed and embedded tissues. However, some organelles, including the plasma membrane, showed differences in size and detail compared to conventional preparations. In experiments using rat liver labeled with 14C-palmitic acid, little labeled lipid was extracted by glutaraldehyde fixation, and over 90% of the labeled lipid was retained during GACH embedding. Analysis of lipid organic phosphate confirmed this result. Under similar conditions, tissues dehydrated with ethanol, hexylene glycol, or acetone retained little lipid label. GACH embedding avoids density due to osmium tetroxide fixation and offers advantages for fine structural localization of specific components in biological membranes.