The interior is on the Latin cross plan with a nave and two aisles divided by columns in granite from Elba, an apse and a dome on the crossing with the transept. It houses a 13th-century "Crucifix" on panel, frescoes by Buonamico Buffalmacco and a "Madonna with Saints" by Turino Vanni (14th century), but most of all a 2nd-century Roman sarcophagus used as medieval tomb. The relief on this sarcophagus was used as a model by both Nicola Pisano and his pupil Arnolfo di Cambio