AGNUS DEI. - A liturgical formula introduced into the Mass by Pope Sergius I, following an ancient Greek usage, with direct reference to the expression of John the Baptist (Jn 1:29). It was to be sung with the response: Miserere nobis, repeated several times during the solemn fraction of the consecrated bread. Since this solemnity was reduced in the 11th century to the celebrant alone, the chant was limited to three invocations, the third with the response: Dona nobis pacem. In Requiem Masses, however, according to an older usage of the Ambrosian rite, it is repeated: Dona eis requiem, with the addition of sempiternam the third time in the Roman rite, and et locum indulgentiae cum sanctis tuis in gloria in the Ambrosian rite.
The same formula of the A. D. is already found almost identical in the Gloria in excelsis (q.v.), a hymn which, according to the Constitutiones Apostolorum (VIII, 13, 13), was to be said in a shorter form by the people immediately before Communion. From this usage, perhaps, the Ecce Agnus Dei etc. was introduced into the Roman rite before distributing Communion to the faithful: a formula which, moreover, is found identically in the ancient liturgy of St. James. The same invocation of the A. D. is already found as early as the 7th century (Ordo of St. Amand) at the conclusion of the litanies, both in the liturgy and privately.