The chemical nature of the electron-dense areas appearing on the organic matrix during the early stage of shell regeneration in Helix has been ascertained. These areas of 500–5000 Å are made up mainly of acid mucopolysaccharides as detected by thorium staining. When treated by 1% phosphotungstic acid (PTA) for a short period, these electron-dense areas took up the stain, suggesting the presence of mucoprotein and glycoproteins, and are probably the nucleation sites for calcification because small crystals of CaCO3 appear with them. The small crystals join to form larger ones. Crystals grow presumably by dendritic growth, and eventually form a calcified layer. Electron diffraction studies on these crystals show the presence of aragonite (type present in the normal shell) and calcite.