IBM is claiming that Informix is using technology that IBM has patented in at least five of its database products, including its Informix-Online Dynamic Server.
IBM is seeking a court injunction preventing Informix from using the patents and monetary damages, according to the complaint filed in Delaware's U.S. District Court.
Informix ranks third in the database market in overall sales, behind market leader Oracle and second-place IBM, according to International Data Corp.
Informix executives refute the allegations.
"We've had sporadic discussions with IBM for approximately 2 1/2 years, and on each occasion, we have carefully analyzed their claims and have concluded that the claims are absolutely without foundation," Informix attorney Gary Lloyd said.
IBM representatives disagreed.
"We've been talking with Informix?about these issues, and about them paying a fair and reasonable licensing fee to use these patents," an IBM spokeswoman said. "This lawsuit is a rare step for IBM, but after more than a year of discussion, we felt we needed to move to that step."