PHILADELPHIA (CNN) -- A legal battle over a law designed to keep sexual material away from children stalled Wednesday on the issue of whether financial details of Web-based companies should be made public.

More than a dozen plaintiffs led by the American Civil Liberties Union (ACLU) are seeking a preliminary injunction against the Child Online Protection Act (COPA) in U.S. District Court.

COPA is Congress' second major effort to protect children using the Internet. The law would require commercial Web sites to collect a credit card number or some other access code as proof of age before an Internet user could view material deemed "harmful to minors."

Violators could face up to six months in jail and a $50,000 fine.

The defendant in the case, the U.S. Justice Department, may use the Web companies' financial information to prove its argument that compliance with the new law is not as prohibitive as opponents claim.

ACLU attorneys asked Judge Lowell Reed on Wednesday to close segments of the hearing so that closely guarded "trade secrets" from the plaintiffs, including Cnet Inc., ArtNet and the online magazine Salon, are not made public.

The Justice Department is opposed to closing any part of the proceedings.

Reed said he would hold a special hearing on the issue if the ACLU and Justice Department could not reach some accord to allow testimony to proceed.

First attempt at law failed

The Supreme Court struck down the previous Communications Decency Act of 1996 on First Amendment grounds.

COPA is a more narrow law, applying the age identification access requirements only to commercial sites.

Before the hearing stalled over financial information, the ACLU's first witness, Donna Hoffman, a professor of management at Vanderbilt University, was called to the stand.

Hoffman, an expert on so-called e-commerce, told the court COPA could endanger growing Internet-based businesses, which saw revenues top $10 billion in 1997, even though many have yet to turn a profit.

She also testified that 75 percent of Web users would not give up personal information to gain access to a site.

"It's not inconceivable that some of these Web sites would go out of business," Hoffman said.

The Associated Press and Reuters contributed to this report.