From the Preface to the New American Bible Revised Edition of the New Testament:
“The Greek text followed in this translation is that of the third edition of The Greek New Testament, edited by Kurt Aland, Matthew Black, Carlo Martini, Bruce Metzger, and Allen Wikgren, and published by the United Bible Societies in 1975. The same text, with a different critical apparatus and variations in punctuation and typography, was published as the twenty-sixth edition of the Nestle-Aland Novum Testamentum Graece in 1979 by the Deutsche Bibelstaftung, Stuttgart. This edition has also been consulted. When variant readings occur, the translation, with few exceptions, follows the reading that was placed in the text of these Greek editions, though the occurance of the principal variants is pointed out in the notes.”
I hope this is helpful.