Sienkiewicz, who in Quo vadis strove to render as accurately as possible the topography of ancient Rome—knowledge of which was increasing dramatically thanks to archaeological finds, especially in the catacombs investigated from 1842 by Giovanni Battista De Rossi—as a rule did not disclose his modern authorities. The only glimpse at the novel’s eruditional aspect is his retort to a charge, repeated in the Polish weekly Słowo after unnamed Roman archaeologists, of having committed an anachronism locating St Peter’s first teaching in Ostrianum, the cemetery founded in the third century, in which he emphasized that some leading Roman archaeologists date it to the first century. Neither party realized that they had different cemeteries in mind. Sienkiewicz’s critic thought that he followed De Rossi’s universally accepted identification of Ostrianum with Coemeterium Maius on Via Nomentana, founded in the third century; yet Sienkiewicz surely identified it with the newly discovered ‘hypogeum of the Acilii’ in the Cemetery of Priscilla on Via Salaria, seemingly datable to the late first century. From whom did he get this identification if its acknowledged author, Orazio Marucchi, five years after the novel’s publication still identified Ostrianum with Coemeterium Maius and was unaware that Sienkiewicz located it in the Cemetery of Priscilla? This chapter tries to find out who may have been Sienkiewicz’s informant and why the location of Ostrianum figuring in Quo vadis published in 1896 was first proposed in a scholarly publication only in 1901, by the archaeologist who a few months before had still held the rival view.