150S

The 150S marked the return to the 4-cylinder engine for racing in 1.5-litre class. One of the car's main attributes was that it was easy to maintain by the privateer. It was another short-stroke design, revving to 7,500 rpm and giving 140 bhp. The 4-speed-gearbox was replaced by a 5-speed unit on some cars. The new tubular chassis featured a very short wheelbase, 2.15 metres at first, later stretched to 2.25 m. The bodies were built by Celestino Fiandri and Medardo Fantuzzi.

The car was designed by Vittorio Bellentani, but the project was taken over by Giulio Alfieri, who had to cure initial problems with the roadholding and handling as well as with cracking differential castings. These teething troubles were forgotten when Jean Behra won the 500-km-race at the Nürburgring in August 1955 against a competitive field of 13 Porsches.

Of the 25 cars made, several were later updated with a 200S engine for use in the 2-litre-class. Among the original owners were Argentine Alejandro de Tomaso as well as his later wife, the American Isabelle Haskell. A 150S engine was used in Formula 1 in 1961, powering the English Emeryson car. A single 150S was bodied as a Grand Touring roadster with an eye on production; this was discarded in favour of the larger 3500 GT.