Ben Spies has announced his retirement from racing after persistent injury problems left the Ducati rider with concerns over his physical ability.
The 29-year-old American managed just two races this season before he crashed out at Indianapolis in August which left him needing shoulder surgery.
Ex-World Superbike champion Spies has reached an agreement with Ducati over his contract, which had a year to run.
"The time has come to stop and I do so with great sadness," Spies said.
"I never dreamed that I would reach the level of success that I have over the past 20 years of racing.
"But I am tremendously disappointed that I have not been able to fulfil my personal goals and team goals with Ducati."
Spies injured his right shoulder a year ago in Malaysia while racing for Yamaha, missing the last two races of 2012 and the start of pre-season testing for the factory-supported Pramac Ducati team.
He suffered chest pains at a race in Texas in May and missed a string of races after that before the latest setback at Indianapolis in August.
Ducati MotoGP project director Paolo Ciabatti added: "We really hoped he would fully recover and continue to race for us in the future.
"However, we understand the reasons for his decision and respect them."