Leoncio was the sixth of eleven children of the couple Isaac and Clara Basbaum, immigrants from Bessarabia, who came to live in Pernambuco. The couple opened a jewelry and gold workshop, where Leoncio worked from an early age. He moved to Rio de Janeiro in 1924, when he enrolled in the Faculty of Medicine of the Federal University of Rio de Janeiro. Although he was a graduate of the medical school, he developed and acted more like a communist activist, political activist, writer and professor, who also dedicated himself to the study ofthe Brazilian reality. After joining the party, he was one of the founders of the Communist Youth and a member of the executive committee (CCE) of the Brazilian Communist Party. He was an important figure in the Brazilian communist militancy, as he was part of the leadership of the Union of Graphic Workers (UTG), delegate to the First Brazilian Congress of Communist Youth, where he positioned himself as anti- militarist, delegate to the 5th International Congress of Communist Youth Community, delegated to the First Latin American Conference of Communist Parties (BuenosAires), but also to the RevolutionaryMilitary Committee in 1929. As a journalist, he used press articles to persuade young people to join the revolutionary movement. He is the author of several books urging the population to adopt a new model of social order.