CHICAGO - The nation's largest hot dog makers argued about the meaning of "100 percent pure beef" and the merits of ketchup Monday in a lawsuit over advertising claims stemming from their years of dog-eat-dog competition.

Attorneys for Sara Lee Corp., which makes Ball Park franks, and Kraft Foods Inc., which makes Oscar Mayer, superimposed giant hot dogs on a courtroom screen as they delivered opening remarks in a case that could clarify how far companies can go when boasting about their products.

"There's never been anything of this scope . . . in the entire history of hot dogs," Sara Lee's attorney, Richard Leighton, said about what the company says is Kraft's false and deceptive ad campaign that claimed Oscar Mayer wieners were the best-tasting franks.

U.S. Magistrate Judge Morton Denlow, who will decide if either company broke false advertising laws, couldn't resist a note of levity as he cast his eyes at the attorneys and proclaimed, "Let the wiener wars begin."

The legal dog fight began when Sara Lee filed a lawsuit in 2009, singling out Oscar Mayer ads that brag its dogs beat Ball Park franks in a national taste test. Leighton argued the tests were deeply flawed and gave as an example that the hot dogs were presented to participants without buns or any condiments, such as ketchup.

"They were served boiled hot dogs on a white paper plate," he told Denlow. As a result, Leighton said, Sara Lee's hot dogs may well have tasted too salty or smoky when consumed sans buns.

Among other flaws, he went on, was a rule barring anyone who ever worked in a factory from taking the test.

"You may be excluding blue-collar workers," he said. "And they're big hot-dog eaters."

Kraft filed a countersuit later in 2009, accusing Sara Lee of running ads for Ball Parks with the tagline "America's Best Franks" based on an award from ChefsBest, a food-judging organization based in San Francisco.

The other focus of the trial is Kraft's claim that its Oscar Mayer Jumbo Beef Franks are "100 percent pure beef." Sara Lee says the claim is untrue, that it cast aspersions on Ball Park franks and damaged their sales.

But Kraft's attorney, Stephen O'Neil, told the judge the 100 percent beef tag was never intended to suggest there weren't other ingredients - like water, salt and various spices. It was only meant to convey that the meat that was used was all beef, he said.

That stress was designed to counter lingering impressions that hot dogs contain suspect, "mysterious meats," he added. And he said it defied common sense to argue that consumers might take the label as meaning that the one and only ingredient was beef.

"If there was nothing but beef, it wouldn't be a hot dog," he said, "It would be a hamburger."

Denlow let slip that, according to his own personal tastes, neither Oscar Mayer nor Ball Park are top dog.

"I already have my favorite . . . and it's none of the brands on trial," he told attorneys. He said he may reveal which one it is - but only after a ruling.

The trial is expected to last about two weeks.