Leo 164 was a priest at Rome ("Leonem presbiterum sanctae Romanae aecclesiae"); in 866 he accompanied the bishop of Ostia, Donatos 2, and the deacon Marinos 15 on a mission (they are called both legati and missi) sent by the bishop of Rome, Nicolas I (Nikolaos 28), to Constantinople to announce Nikolaos 28's judgment on the affair of the patriarchs Photios 1 and Ignatios 1; they travelled through Bulgaria (cf. also Formosus 1 and Paulos 57) but were held up at the frontier with Byzantium by a local official, Theodoros 181, who refused to allow them to go further and subjected them to insults and abuse; they remained there for forty days, until they learned that the emperor had forbidden them to enter Byzantine territory; they then returned to Rome to report what had happened: Lib. Pont. 107. 70-72.

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