IBM, the world's largest information-technology company, on Wednesday said it agreed to buy Internet Security Systems Inc. for $1.3 billion, continuing an acquisition drive to fuel growth.

International Business Machines Corp. said it will pay $28 a share for the company, which helps customers protect against Internet threats across networks, desktop computers and servers. Internet Security Systems shares closed at $26 on Tuesday.

Internet Security is a services company that helps companies protect themselves against Internet attacks. IBM will sell those products through its global services unit, the world's largest IT services company.

This is something we couldn't do before because we didn't have the software assets to provide protection against Internet attacks, said Kristof Kloeckner, vice president of strategy and technology for IBM's software group.

IBM said it expects the acquisition to close in the fourth quarter, subject to shareholder and regulatory approvals. The deal is the fourth acquisition IBM has announced in August.

On August 10 IBM said it would buy FileNet Corp. for $1.6 billion, its biggest acquisition in three years and fourth-largest to date.

Earlier in the month the company announced plans to purchase MRO Software Inc. for $740 million in cash and privately held Webify Solutions for an undisclosed sum.