The buzzer has sounded, so brace yourself for round five of the Great Mozzarella Melee.

In the battle thus far, going back to 1979, the dairy lobby has lost each round of an effort to force the Agriculture Department to require frozen-meat-pizza makers to prominently label their product when it contains imitation mozzarella cheese.

The National Milk Producers Federation argues that consumers ought to be told in no uncertain terms when their frozen pizzas are confected with substitutes that contain imported casein, a milk byproduct, and soybean oil. The dairy producers believe that unsuspecting consumers will revolt and demand real cheese on the pizzas. Translation: More money for dairy producers.

But the pizza makers, which include major food processors such as Pillsbury and Quaker Oats, object. They contend that the cheese substitute is more healthful (less fat, less cholesterol) and cheaper for consumers than the real thing. Translation: More money for pizza makers.

This spring, Agriculture's Food Safety and Inspection Service spoke what apparently was the last word on the cheese issue. After five years and more than 5,000 public comments, it decided to drop a proposal to require more real cheese and tighter labeling on the frozen-meat pizzas.

Undeterred, the dairy producers have gone now to Capitol Hill with a new recipe for pizza magic: a way deficit-haunted legislators can save about $50 million by requiring frozen pizza packages to say so more explicitly when they contain imitation mozzarella.

By the milk producers' reckoning, when consumers turn up their noses at substitutes and demand real cheese, more milk products will be sold and Uncle Sam will be spared from having to buy an estimated $50 million in dairy surpluses.

And not at all surprisingly in Congress, where a willingness to listen to the dairy producers is on a par with the hunger for budget cuts, they're listening.

The labeling proposal is under consideration by the Senate Agriculture Committee, which has about two weeks to come up with ways to meet fiscal 1988 budget requirements by cutting $1.2 billion from agricultural spending.

An aide to Chairman Patrick J. Leahy (D-Vt.) said the committee will not make a decision on the labeling idea until it hears from the Congressional Budget Office, which has been asked to calculate how much, if any, savings would accrue from diminished use of cheese substitutes.

The House Agriculture Committee is reported to be moving toward adopting the dairy producers' proposal and the Energy and Commerce Committee also is toying with the idea although no final decision has been made.

The Energy Committee, equally eager to claim some budget savings, is invoking jurisdiction because it oversees the Food and Drug Administration, which regulates meatless frozen cheese pizzas (unlike the Agriculture Department, the FDA requires labeling for the substitutes).

"We've been working on the House side, because it comes up there first," said James Barr, chief of the federation. "It looks favorable . . . . We think we've got a shot at getting it in the House reconciliation package . . . . Our numbers show a cost differential between imitation and real cheese of about five cents on a small pizza to 12 cents on a large pizza. It's really a profit issue with the pizza companies."

The Ad Hoc Pizza Standard Rulemaking Group (note the labeling), with mushrooming support for its position from the American Heart Association (health reasons), consumerists (cost reasons) and labor (job reasons), is blitzing lawmakers with the other side of the story.

Consultant Carol Tucker Foreman, the assistant secretary of agriculture in 1979 when the pizza-cheese flap began, is leading the charge for the frozen pizza industry. She described the dairy move as "a new way to pick consumers' pockets" as well as a disregard by the farmers of various governmental dietary recommendations for less fat and cholesterol.

To top it off, Foreman said, the budget proposals "will not put one dollar into the hands of dairy farmers" because, according to one economic analysis, while consumer costs for frozen pizza would go up, consumption would go down dramatically and other farm sectors would suffer. Soybean oil use would drop, substitute cheese use would drop, meat sales would go down, she said.

There's the bell -- round five is under way.