Invisible tags sometimes called Web bugs are widely used in HTML e-mail by marketers and others to detect whether an individual has opened an e-mail message. The Congressional Privacy Caucus has announced plans to hold hearings to investigate the use of Web bugs later this month. Mr. Smith said that it was now clear that JavaScript could be used to create a more powerful Web bug so that not only can someone find out when a message is read, but also what is being said about it.

Until recently, almost no one but computer specialists had heard about Web bugs, often known in the trade as 'clear GIFs.' But with exploding interest in target marketing on the Internet — and several companies poised to begin identifying computer users by name — Web bugs have suddenly drawn the attention of advertisers, government officials and privacy advocates.