I.B.M., the holder of the largest patent storehouse in corporate America, often finds itself defending itself against patent claims from aggressive plaintiffs, seeking a payout.

But I.B.M. became the aggressor yesterday when it filed two lawsuits against Amazon.com, contending that the company has built its big online retail business on technology developed by I.B.M.

The suits, filed in two federal courts in Texas, seek unspecified damages, but I.B.M. says that Amazon’s infringement is broad, suggesting the potential for a sizable judgment or settlement. An I.B.M. spokesman said the figure could run to hundreds of millions of dollars.

“These patents are core to modern electronic commerce,” said John E. Kelly III, senior vice president for technology and intellectual property at I.B.M. “Most if not all of Amazon’s business is built on top of this technology.”