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CONCORD -- NASCAR Chairman Brian France won't reveal what caused Jeremy Mayfield's positive drug test despite a growing number of drivers saying that the information needs to be released.

"If we released it, I think it would provide credibility to (testing)," Jeff Burton said.

Although France called Mayfield's positive test "serious'' and hinted at what caused it, he cited privacy issues for not divulging the information.

"There's nothing to be gained by disclosing exactly what the substance that tested positive in Jeremy's case or anyone else's case," France said Friday afternoon at Lowe's Motor Speedway. "If we thought there was a benefit, we would probably rethink that. What's important to know is when it's serious."

France, though, suggested Mayfield failed the test because of either a performance-enhancing or recreational drug.

"It," France said of a prescription or over-the-counter medicine, "would not invoke an indefinite suspension of the kind that Jeremy or others have gotten."

Mayfield issued a statement last week blaming a reaction between a prescription medicine and over-the-counter medicine for the positive test. He has not issued a statement since.

France also fought criticism by some drivers who say they're not aware of any list of banned substances. France said that NASCAR had a "broad list" of banned substances.

"The drivers &ellipses; are happy to look at that list," France said. "We show it to them."

Those unaware of such a list France alluded to are afraid of what could cause a positive test.

"It's just a little bit scary," Mark Martin said. "It's our careers, man. I'm sure that we're all over-reacting a little bit, but certainly nobody wants to be caught up in that."

France also defended NASCAR allowing Mayfield to drive at Darlington after Mayfield's first sample had tested positive earlier that week. Those tested can request to have a B sample -- part of the original urine sample -- tested to verify the first sample's results.

Later Friday, NASCAR amended its timeline and stated that series officials were not informed of Mayfield's positive test until after he practiced at Darlington.

"We would have been within our rights if we thought Jeremy or any driver was impaired, based on the fact we had some new knowledge, but that wasn't the case," France said.

"Then we determined the obvious: that it was not a completed test until you give the member a right to have the second test finished, which we did. We did that as fast as we could with the laboratory and still be thorough."

Mayfield failed to qualify at Darlington. Series officials said they were notified of Mayfield's B sample also testing positive about seven hours before the start of the Southern 500 last weekend.

Contact Dustin Long at 373-7062 or dustin.long @news-record.com

Today's lineup

(At Lowe's Motor Speedway; Concord)

Lap length: 1.5 miles

(Car number in parentheses)

1. (48) Jimmie Johnson, Chevrolet, 133.426 mph.

2. (2) Kurt Busch, Dodge, 132.584.

3. (17) Matt Kenseth, Ford, 131.51.

4. (88) Dale Earnhardt Jr., Chevrolet, 131.42.

5. (5) Mark Martin, Chevrolet, 131.02.

6. (24) Jeff Gordon, Chevrolet, 130.287.

7. (96) Bobby Labonte, Ford, 130.271.

8. (9) Kasey Kahne, Dodge, 129.989.

9. (29) Kevin Harvick, Chevrolet, 129.869.

10. (09) Brad Keselowski, Chevrolet, 129.613.

11. (16) Greg Biffle, Ford, 128.046.

12. (39) Ryan Newman, Chevrolet, 128.044.

13. (18) Kyle Busch, Toyota, 127.805.

14. (99) Carl Edwards, Ford, 118.419.

15. (14) Tony Stewart, Chevrolet, 117.572.

16. (31) Jeff Burton, Chevrolet, 112.694.

17. (33) Clint Bowyer, Chevrolet, 111.433.

18. (11) Denny Hamlin, Toyota.