This article analyses an early stage of legal adoption in Argentina: the sixties. It aims to understand the relationship between formal and informal adoption practices in a country where customary practices of adoption had existed since colonial times. Through the privileged analysis of legal files, I inquire into the mechanisms instituted by the legal system and the ways in which the family origins of the adopted children were assessed. I argue that the process of institutionalisation of adoption has its basis in the legalisation of existing practices, in a context in which formal and informal adoption systems coexisted and were implicated in a process of mutual feedback.