Zbikowski fought Saturday at the WinStar World Casino in Thackerville, knocking out Blake Warner of Oklahoma City in the first round and improving his record to 4-0.
Sixteen fighters underwent drug tests and five of them came back positive. A tribal spokeswoman says the Chickasaw Nation Gaming Commission suspended those fighters for 45 days.
Zbikowski's manager, Mike Joyce of Chicago, says he's been told Zbikowski tested positive for traces of a substance found in marijuana.
Joyce says Zbikowski took another test Tuesday at a Chicago lab and it came back negative. Joyce says Zbikowski has never tested positive for banned substances.