Marquardt made his UFC debut on August 6, 2005, live on Spike TV, headlining the inaugural UFC Ultimate Fight Night card in Las Vegas, Nevada. Although Marquardt earned a unanimous decision victory over UFC veteran Ivan Salaverry, a post-fight drug test revealed high levels of nandrolone, an anabolic steroid, in Marquardt's system which led to the Nevada State Athletic Commission filing for his suspension. Marquardt maintains that he only used over-the-counter supplements in preparation for the fight. Marquardt was initially suspended for six months and assessed no fine. Marquardt's suspension was reduced to five months and he was reinstated in January 2006.