The chapel of St. Aquilino, part of the basilical complex of St. Lorenzo in Milan, has a sculpted portal of the Roman age. Following a request by Soprintendenza Archeologica della Lombardia, the portal was subjected to an historical, technical, and conservation study. Different techniques were used, including light and electron microscopy, x-ray diffraction and fluorescence, and porosimetry. The analyses showed that the portal is sculpted in a thin slab of Carrara marble and is covered in many places by a crust of gypsum. The different kinds of mortar, used for various restorations, indicate that the portal is the same age as the mosaics of the chapel (4th-5th century AD). The portal was cleaned with distilled water and Attapulgite packs, and then consolidated with methylphenyl silicone resin. The fragments of the epistyle were sealed, and fixed with Teflon pins.