AOL Time Warner launched a legal battle against Microsoft last week while dealing with rumors that it was buying Linux developer and distributor Red Hat.
Netscape Communications, a subsidiary of AOL, filed a seven-count suit against Microsoft, alleging the software company harmed it by promoting the Internet Explorer Web browser at the expense of the Netscape Navigator browser.
AOL said in a statement that its case is based on the same anti-competitive conduct that formed the basis of the governments landmark antitrust case against Microsoft.
Microsoft countered with a stinging attack against AOL, accusing it of trying to undermine the proposed antitrust settlement it has reached with the Department of Justice and nine states.
The suit follows findings last year by a federal judge that Microsofts behavior violated antitrust laws and harmed Netscape.
The rumored acquisition of Red Hat by AOL was seen as the latest salvo in a battle between the latter company and Microsoft, which have clashed over such technology as instant messaging and media players.
AOL and Red Hat had no comment on rumored negotiations.