The article deals with several problems concerning the parentage of D. Brutus Albinus, one of the leaders of the conspiracy against Caesar: the identity of his adoptive father, his relationship with Sempronia mentioned by Sallust and the lineage of this Sempronia. The results of the research make it possible to advance a hypothesis on the source of Cornelius Nepos’ problematic evidence about Gaius Gracchus’ relationship with Junii Bruti.