In 1967, he set the Latin words of the liturgical Agnus Dei, a part of the Mass, for mixed chorus with optional organ or piano accompaniment.

After reflecting the history and pointing out the Adagio's associations with mourning, nostalgia, love and passion, qualified as "sentimental Romanticism", he writes about the choral setting: "Barber brought to the surface the work's sense of spirituality."

The music is dominated by a melody, first presented by the soprano, which begins on a long note and then undulates in even rhythm and diatonic steps, a melisma of two measures on the words "Agnus Dei".

The repetition of the call "Agnus Dei" is set as variation of the beginning, intensified by upward leaps of fifths and octaves, and by the solo soprano reaching the highest note of the piece, C-flat.

Then the alto takes over the melody, marked "più f[orte] sempre espressivo" (somewhat stronger and always expressive), while the soprano sings "miserere nobis" (have mercy on us) for the first time on a counter-melody.

molto espressivo" (soft but growing, very expressive), while the three upper undivided voices sing "dona nobis pacem" (give us peace) the first time.

In measure 35, the tenor takes over the melody, all parts are marked "with increasing intensity", soon the soprano gets the melody, interrupted by the alto moving in octaves (including a top A flat in measure 46 for first altos), then finally the soprano leads to the climax on the words "dona nobis pacem", ending in long chords, fortissimo, in extremely high register for all parts, followed by a long general break.