The aim of this research is to wonder about the possibility of Lactantius having read the Libri ad edictum of Ulpian. This question arises regarding the expression ius fraternitatis, which Lactantius uses to represent an idea of fraternity among all human beings that implies some obligations towards all men. However, this expression has a history: before Lactantius, it appears in two more texts as well. The first one is a fragment of Tertullian’s De cultu feminarum and the second one, a fragment belonging to the commentary of Ulpian to the edict of the praetor, which has arrived to us via Justinian’s Digest (D. 17. 2. 63 pr.). Whereas the expression in Tertullian’s text seems to have been deprived from its most technical legal content, an analysis of the content of the expression in the Ulpianian fragment reveals that it is coherent and sheds light on Lactantius’ thought. This consistency, together with some other reasons that are analysed in this article, make us think that Lactantius probably accessed and read the Ulpianian text.