According to UFC vice president of regulatory affairs Marc Ratner in an e-mail to MMAWeekly.com, six random fighters plus the main event fighters were drug tested and Quieroz was the only fighter who came back with a positive test.
Quieroz lost his fight at UFC 120 by submission to British heavyweight Rob Broughton.
The test results showed Quieroz was positive for Stanozolol, which is an anabolic steroid.
The Brazilian was notified of the results and has been released by the UFC.
Ratner also confirmed that all of the other fighters tested came back negative for performance enhancers and drugs of abuse.