With the fall season's top pilots already leaked to file-sharing Web sites, industry experts are renewing their anti-piracy efforts to try to halt illegal trafficking of popular shows.

During the past few weeks, nearly a dozen fall pilots have been made available via peer-to-peer servers. The pilots were seemingly ripped from DVD screeners and are of relatively high quality, akin to the streaming video offerings on the networks' own Web sites. The pilots join the thousands of previously aired television shows already available for illegal download.

Many of the peer-to-peer sites (or "torrent" sites, named after the popular file-trading software client BitTorrent) offering the programs are already being pursued by the legal representatives of the Motion Picture Association of America, which also represents television studios in the anti-piracy fight. But in practical terms, experts said once the files are on torrent sites, they become nearly impossible to track.

An MPAA study last year found that movie piracy cost $6.1 billion in lost annual revenue, while an industry anti-piracy expert said movie and television studios are spending more than $100 million per year in efforts to stop illegal content distribution.

The current television industry strategy for fighting piracy is threefold: Embrace legal, centralized hubs for online content distribution (such as iTunes and NBC/News Corp.'s upcoming video service) that rival those of piracy sites. Take legal action against illegal content distributions to marginalize their impact. Work with Internet service providers and technology manufacturers to add anti-piracy measures.

Though the MPAA officially leads the charge, Warner Bros., NBC Universal, News Corp. and The Walt Disney Co. all have in-house divisions fighting piracy.

"Online piracy can be vastly reduced," said Rick Cotton, executive VP and general counsel for NBC Universal. "The challenge is to get it away from where it is now, which is the complete Wild West, without any effective technology blocks."

One promising new technology is Audible Magic, software that recognizes copyrighted content even from grainy video signals. Social networking Web site MySpace agreed to add the software earlier this year in an effort to keep movies, shows and music from being pirated through its service. After pressure from studios, last week YouTube agreed to add the software as well.

The MPAA and studios expect several major ISPs to embrace the software next, which would help curtail illegal downloading at the user level, rather than having to pursue outlaw torrent sites.

"The studios and networks continue to ratchet up policies and procedures to protect their content from illegal distribution," said Darcy Antonellis, senior VP, worldwide anti-piracy operations, at Warner Bros.

Pirate sites have an advantage, however, because they offer one-stop shopping for content, while television networks prefer to offer programs on their own Web sites and through key partners.

In addition to the upcoming NBC/News Corp. video service, several other companies are vying to become one-stop hubs that give consumers a single place to legally view content from multiple networks. Joost started a service in May, and last week BitTorrent announced it will launch a streaming-video service in September.

On the litigation front, while getting content removed from traditional U.S.-based Web sites is relatively easy, shutting down peer-to-peer trafficking is highly difficult. Some of the torrent sites are U.S.-based and others are overseas. Some are run by free-content ideologists rather than profit-minded entrepreneurs, which can make them more difficult to stop.

"Sending a cease-and-desist is like sending a letter to a Colombian drug lord," said an attorney involved in the MPAA's anti-piracy efforts. "But by taking action early, we are trying to allow a legitimate [file-selling] market to take hold."

The attorney said the MPAA's litigation goal isn't to eliminate online trading -- which is impossible -- but to shut down major torrent sites, or at least scare off their advertisers to minimize revenue potential.

The tricky part is that industrywide changes tend to be gradual and cumbersome.

Recently leaked pilots include NBC's "Bionic Woman," "Chuck" and "Lipstick Jungle"; ABC's "Pushing Daisies" and "Cavemen"; Fox's "Sarah Connor Chronicles"; and The CW's "Reaper."

The videos are typically watermarked with an onscreen code, but only by distribution categories. Marking each individual DVD, which would allow networks to identify the person responsible for the pirating, can be cost-prohibitive. So if a pilot leaks, the studio can typically only tell if the DVD was, for example, a critics screener or from the public relations stock -- not which person received it.

That the most buzz-heavy pilots are the same ones leaked has caused some to suspect that networks or studios leak the pilots themselves.

But industry anti-piracy executives uniformly denied leaking content, noting it runs against every effort studios are making to stop the problem. Mr. Cotton called the notion of networks and studios leaking content "ridiculous."

"NBC.com and others are increasingly streaming shows," he said. "Everyone is fully engaged in the effort to utilize the capability of digital media. Against the background of these very aggressive efforts, we're certainly not going to facilitate pirating those same shows."

Studio sources said they would panic if in-season episodes popped up on file-sharing services before their air date -- a situation that would indicate a leak from inside the network or studio. But the pilot screeners have been distributed so widely that online leaks were considered inevitable.

"The number of people who view them is not great enough to hurt the premiere rating," said Vince Manze, president of program planning, scheduling and strategy at NBC. "And the promotion and word-of-mouth buzz has helped build anticipation for these shows. Unlike a movie, we're selling 22 episodes -- not just the premiere."