U.S. antitrust regulators are investigating possible anti-competitive practices at the American Stock Exchange (search) and Chicago Board Options Exchange (search), according to the annual report of the National Association of Securities Dealers (search).

The NASD, which agreed to sell Amex to that exchange's members, said Amex on June 16 received a civil investigative demand from the Department of Justice's antitrust division to determine whether there has been a violation of the Sherman Act (search).

The probe concerns "conduct, activities or proposed action of the following nature: product allocation agreement" between the Amex and CBOE, the NASD said.

There are no further details on whether the probe is possibly related to the listing of equity or index options.

The NASD said Amex "believes that no product allocation agreement between Amex and CBOE has ever existed and that the subject of the investigation is without foundation, at this very early stage of the proceedings." It said Amex is working out a schedule to comply with Department of Justice requests.

The Amex can not comment on investigations, a spokeswoman said, adding: "We are cooperating fully with the Department of Justice."

The CBOE, one of the two largest equity options exchanges, said in a statement that it was notified of the probe and is cooperating with the Department of Justice.

"CBOE believes that its conduct has been appropriate and lawful," the statement said.

The Amex sale, which in March won approval from the buyers, Amex Membership Corp., requires Securities and Exchange Commission (search) approval, the NASD said in its annual report.