Mainframe computer vendor Platform Solutions (PSI) has countersued IBM, saying the giant company should not have accused it of infringing patents when it created computers that allow customers to run IBM's System z operating system and software on mainframes from other vendors.

IBM sued the Sunnyvale, Calif., company in November for patent infringement and breach of contract, since PSI is a licensed z/OS user. Now PSI has filed its own lawsuit, alleging that IBM has committed antitrust violations, unfair competition and business torts, according to papers filed Friday in the U.S. District Court for the Southern District of New York.

PSI's computers provide the only alternative in a market where IBM has held a monopoly position for decades, PSI vice president of product management and marketing, Christian Reilly, said in a release.

"IBM's lawsuit against PSI is part of a concerted effort to eliminate competition for mainframe computers, and its refusal to provide the operating systems needed to run mainframe computers is PSI computers is blatantly anticompetitive," Reilly said.

IBM did not respond to a request for comment.

PSI plans to launch a line of "open mainframe computers" later in 2007 that can run IBM's z/OS as well as other data center operating systems like HP-UX, Linux and Windows. PSI is now testing the design with corporate beta users, and has demonstrated it at trade shows such as the IBM users' group SHARE.

PSI hopes to sell the mainframes to corporate and government users who need to perform high-volume and mission-critical data processing needs, including matters such a billing, accounting, order entry, record keeping and transaction processing, the company said.

In addition to denying IBM's patent infringement claims, PSI's lawsuit charges that IBM has now turned to illegal tactics, such as barring its software customers from using other vendors' hardware.

"Contrary to IBM’s assertions, IBM has and is engaged in anticompetitive, deceptive and tortious acts intended to eliminate competition and prolong its monopoly in the worldwide market for mainframe computers compatible with IBM applications and software," PSI said in its lawsuit.

"Until 2001, IBM’s ability to abuse its monopoly power was limited by the existence of competition from developers of other IBM-compatible mainframes such as Amdahl and Hitachi. Since the decisions by Amdahl and Hitachi to exit the mainframe market, IBM’s market power in the IBM-compatible mainframe market has grown year by year."

The IDG News Service is a Network World affiliate.