Work on the new gas distribution network in the metropolitan area of Rome, was undertaken with supervision of the Archaeological Superintendency of Rome. The excavation of a trench for the laying of a gas pipe situated between Piazza Ss. Apostoli and Via del Vaccaro brought to light walled remains belonging to a medieval building. These were two supporting walls in opus listatum, with a part of the related floor paved in opus signinum, and a travertine block enclosed below the east section of the north wall, of a building whose original dimensions could not be determined. In fact, previous work by the gas company had already seriously damaged the structures and caused the loss of the entire stratigraphic sequence and any useful dating elements. However, the structure of the walls in regular _opus listatum_, in particular the large dimensions and squared form of the tufa blocks do seem to be typical characteristic of _opus listatum_ walls of the medieval period. Close comparisons can be made with the structures forming the central part of the exterior façade of the _atrium_ of the church of S. Clemente (late 11th-beginning of the 12th century). The building seems to have had at least two phases as shown by the raised level of the cocciopesto floor and the travertine block with respect to the original level of the _spicatum_ paving related to the walls. Its site near the church of the Ss. Apostoli, may link it to the property of the family of Alberico “princeps et senator omnium Romanorum” and the Counts of Tuscolo, which then passed to the Colonna, who as is known from written sources, were supposed to have lived “iuxta sanctos Apostolos” in the 10th and 11th centuries.