WASHINGTON - U.S. companies will need to keep track of all the e-mails, instant messages and other electronic documents generated by their employees thanks to new federal rules that go into effect Friday, legal experts say.

The rules, approved by the Supreme Court in April, require companies and other entities involved in federal litigation to produce "electronically stored information" as part of the discovery process, when evidence is shared by both sides before a trial.

The change makes it more important for companies to know what electronic information they have and where.  Under the new rules, an information technology employee who routinely copies over a backup computer tape could be committing the equivalent of "virtual shredding," said Alvin F. Lindsay, a partner at Hogan & Hartson LLP and expert on technology and litigation.