His older brother, Giovanni Battista Possevino (1522–1552) arrived in the mid-1540s in the Rome of Paul III Farnese, first in the service of the Mantuan cardinal reformer Gregorio Cortese, then of the papal "cardinal nipote" Alessandro Farnese (cardinal) and finally of cardinal Ippolito II d'Este. In 1549 at seventeen Antonio came to study with his brother in Rome and met the leading intellectuals at the Renaissance court of pope Julius III (1550–1555), the patron of Giovanni Pierluigi da Palestrina and the builder of Villa Giulia. These included Fulvio Orsini and Paulus Manutius. In 1553 he published posthumously the "Dialogo dell'Honore" of Giovanni Battista who died not yet thirty. In Rome he dedicated the "Centones ex Vergilio" published under the name of Lelio Capilupi, to the French poet Joachim Du Bellay and in 1556 in "Due Discorsi" he defended his brother against accusations of plagiarism and defended the writings of Giovanni Battista Giraldi.