THE odontoblastic processes of human dentin are projections of the odontoblast cytoplasm into spaces in the dense collagenous, highly mineralized dentin matrix1. These projections can be up to several mm in length resulting in the cytoplasm being a long distance (2,000–3,000µ) from the odontoblast nucleus. The contents of odontoblastic processes in published electron micrographs have shown a varied appearance. In general they display a granular texture but do not contain recognizable cytoplasmic organelles2–4. Most of these specimens were prepared with standard osmium fixatives which have a very poor penetrating ability. We have investigated the ultrastructure of dentin fixed with glutaraldehyde which is particularly favourable for the preservation of membrane systems and has good penetrating ability. A freshly extracted tooth (severed from its blood supply no longer than 8 min) was broken into small fragments with at least one dimension 0.5–1 mm thick. These were immersed in cold fixative containing 1.25 per cent glutaraldehyde in s-collidine buffered to pH 7.4–7.55 and adjusted to a final osmolality of 360 milliosmols. Samples were post-fixed in 4 per cent osmium tetroxide for 48 h and then decalcified in a nitric acid–formalin mixture5 for 12 h. They were then dehydrated in an alcohol series and embedded in ‘Epon’ B (ref. 6). Sections were cut with a diamond knife on an LKB ‘Ultratome’ and examined in an RCA EMU-3-G electron microscope.