A month after Koninklijke Philips Electronics N.V. (NYSE:PHG) sued Zoll Medical Corp. (NSDQ:ZOLL) in a patent infringement case, the Zoll fought back with a lawsuit of its own.

Philips accused the Chelmsford, Mass.-based firm of violating a string of automated external defibrillator patents with its AED Plus and AED Pro devices. Now Zoll, in a lawsuit filed in the U.S. District Court for Massachusetts, is accusing Philips of violating five of its patents with the HeartStart MRx, HeartStart XL, and HeartStart AED lines.

Chelmsford-based Zoll is seeking a jury trial, an injunction against further sales of the allegedly infringing devices, triple damages, interest and legal fees, according to court documents. Philips’ AED operation is headquartered in Andover, Mass.

The spat began June 22, when Philips alleged that Zoll willfully violated 15 patents despite Philips notifying the company of the alleged infringements in November 2008, according to court documents.

As with the Zoll suit, Philips wants a jury trial, an injunction barring further infringement, damages and interest on the alleged infringement, triple damages for the allegedly willful violation of the patents and legal fees.

External defibrillators are big business for both companies. Last fall, each won lucrative contracts from the U.S government to supply the devices, which are used to shock the heart back to beating after a heart attack. Zoll won a $29.8 million contract from the Defense Supply Center in Philadelphia to provide its CCT defibrillators to the U.S. Army, Navy, Air Force, Marine Corps and federal agencies; Andover, Mass.-based Philips Healthcare won a $26.6 million deal from the Dept. of Defense’s Philadelphia supply center to provide its HeartStart MRx defibrillators to the same cast of military and federal agencies.