Sprint Cup Series driver A.J. Allmendinger was suspended indefinitely after his "B" sample urine test came up positive, in violation of NASCAR's substance abuse policy.
Allmendinger, 30, had been suspended temporarily earlier this month for failing his initial drug test that he took the last weekend in June when the series competed at Kentucky Speedway.
NASCAR didn't disclose what Allmendinger tested positive for.
He is in his first year as driver of the No. 22 Dodge for Penske Racing. He replaced Kurt Busch in the car after Busch was dismissed from the team following the 2012 Sprint Cup season.
Sam Hornish Jr., who drives full-time for Penske in the Nationwide Series, was called upon to substitute for Allmendinger in the No. 22 at Daytona earlier this month, arriving at the racetrack moments before the start of the event. Hornish also drove the car July 15 at New Hampshire Motor Speedway.
"Penske Racing is very disappointed with the result of the B sample test and will evaluate its course of action as it pertains to AJ over the coming week," Penske racing said in a statement. "Sam Hornish Jr., will drive the No. 22 Dodge Charger this weekend at Indianapolis and next weekend at Pocono."
Because Allmendinger's "B" sample came back positive, he will have to successfully complete a drug treatment program before he can be reinstated.
Allmendinger is the second Sprint Cup driver to be suspended since NASCAR revised its drug policy prior to the start of the 2009 season.
Jeremy Mayfield failed a drug test in May '09 and has not competed in a NASCAR event since then. Mayfield had continuously lost his legal battle with NASCAR in court.