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.

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.