With Mercedes-Benz gone, Fangio joined Scuderia Ferrari. The cars he and his teammates would race were not conventional Ferraris but the Lancia D50s — transferred from Lancia to Maranello after Lancia's withdrawal in mid-1955 and now running in Ferrari colours under the designation Lancia-Ferrari D50. Stirling Moss, likewise released by Mercedes, joined Maserati as works leader. The Buenos Aires round was the Gran Premio de la Republica Argentina, the fourth running of the event on the championship calendar, run in sunny, hot conditions.
The Buenos Aires grid was composed entirely of Italian machinery. Ferrari entered Lancia-Ferrari D50s for Fangio, Eugenio Castellotti, Peter Collins, Luigi Musso and the Belgian Olivier Gendebien. Maserati countered with Moss, Jean Behra, Jose Froilan Gonzalez, Carlos Menditeguy and Luigi Piotti. Mike Hawthorn made a brief appearance in a Maserati. Private Maseratis, including the shared car of Gerino Gerini and Chico Landi and the entry of Alberto Uria and Oscar Gonzalez, completed the field.
Fangio took pole position in his D50, with Castellotti's sister D50 second. Moss was third in the leading Maserati, Behra fourth, Gonzalez fifth and Menditeguy sixth.
The race opened with the Ferraris and Maseratis settled into a tight order at the front. Fangio's own race ended on lap 22 when the D50's fuel pump failed. Under the period's regulations governing shared drives, Ferrari called Luigi Musso in from competitive running and handed his car to Fangio. The car-share was effectively a team management decision giving the championship leader priority over the second driver.
Fangio re-entered in Musso's car and set the race's fastest lap of 1:45.3 on lap 42. A sequence of mechanical failures then reshaped the order: Gonzalez retired with engine failure on lap 24; Castellotti's gearbox failed on lap 40; Menditeguy broke a halfshaft on lap 42; Collins crashed out on lap 58; and Moss's Maserati engine failed on lap 81, ending what had been a competitive run.
The Musso-Fangio Ferrari took the chequered flag in 3 hours 0 minutes and 3.7 seconds. Behra's Maserati finished second 24.4 seconds behind. Hawthorn brought his Maserati home third, two laps down. The shared Gerini-Landi Maserati was fourth at six laps down, and Gendebien's Ferrari fifth at seven laps adrift. The Uria-Gonzalez Maserati was classified sixth.
The Maserati team lodged a protest claiming Fangio had been push-started following a spin during the race — a breach that could have cost him the result. Both the race stewards and the FIA rejected the protest, confirming the shared Ferrari as the winner.
The victory was shared between Musso and Fangio under the rules governing shared drives. Musso had driven the first portion and Fangio the remainder; the eight points for first place were divided between them, with Fangio also receiving the single point for fastest lap. Behra's six points for second place underlined Maserati's competitive credentials despite the defeat. The result was recorded as Ferrari's 21st championship win and Fangio's 18th career Formula One victory. The next round was the Monaco Grand Prix on 13 May.