NASCAR chairman Brian France has classified Jeremy Mayfield's positive test as "a serious violation" of the sport's toughened drug policy.

France will not reveal what substance Mayfield was caught using in a random drug test May 1. NASCAR indefinitely suspended the driver last week after his backup "B" sample also tested positive.

Many drivers have called on NASCAR to disclose the drug, in part to clear up any confusion over what is banned under the policy.

But France says NASCAR sees no benefit in revealing the substance.