Last week, the band delivered 13 boxes full of legal documents identifying usernames of people who allegedly made Metallica songs available online and demanded that they be blocked from the MP3 music-swapping service.
The music company agreed today but cautioned that "it is possible that users have been mistakenly implicated."
"Napster has blocked all users identified by Metallica as allegedly infringing, based on Metallica's sworn allegations against these usernames," the company wrote on its Web site today. "We intend to fully comply with (federal copyright law) and our policies."
A statement from executives elaborating on the action is expected tomorrow.
Napster's music-swapping program, which allows computer users to open their hard drives and quickly trade MP3 files with thousands of other people online, has thrown a panic into the music industry.
The Recording Industry Association of America sued the company in December, alleging it had contributed to numerous copyright violations. Metallica and rapper Dr. Dre last month filed their own suits against Napster.
But Metallica raised the stakes on the legal battles last week. It hired British copyright protection firm NetPD to sniff out hundreds of thousands of Napster users who had made the band's songs available online though the Napster service and then presented the small software company with these individuals' usernames.
That put Napster in a legal bind: block its subscribers or risk undermining some of the legal arguments that it is using to counter the record industry's court attacks.
Napster has chosen the safer legal path, but it was careful to let its members know that they have a way to appeal their eviction.
"Because of the methods employed by Metallica in assembling its list of usernames, it is possible that users have been mistakenly implicated as infringing the copyrights of songs and recordings originally included on commercially released Metallica albums," the company wrote on its Web site today.
Appealing the action could lead to reactivation of a Napster account, but it also could expose a person to a lawsuit from Metallica, the company said. Napster offered a full explanation of the appeal process on its Web site.
Napster users could face other threats as the music industry becomes more sophisticated about the software company's operations.
One example: NetPD said last week that it had other data about individuals, such as the IP addresses that can identify individual computers on the Net, that it hadn't given to Metallica. That information, if ultimately passed on to Napster by Metallica, could provide a more effective way to block many individuals from using the service.
Other musical acts, such as rapper Dr. Dre, have said they plan to follow Metallica's lead if the band's actions prove effective in dissuading people from trading its songs online.
Metallica's attorneys could not immediately be reached for comment.