Turn over a frozen pizza; no telling what you'll find.

This is why:

Five years ago, the Agriculture Department's Food Safety and Inspection Service (FSIS) proposed requiring the use of more real cheese in frozen meat pizzas and toughening the labeling rules on meat and poultry products containing cheese substitutes.

Some consumer groups, which find cheese substitutes nutritionally acceptable, were not happy. Nor were the pizza makers, including big-time operators like Pillsbury and Jeno's, who rely heavily on substitutes that are made with inexpensive imported casein, a protein byproduct of milk.

But the National Milk Producers Federation, a.k.a. The Influential Dairy Lobby, was understandably elated. Toughen the rules, the federation reasoned, and more domestically produced cheese would go into the frozen pizzas. The casein enemy, which the federation has gone after repeatedly, would be treed.

The cards and letters came flowing in from both sides -- more than 5,500 comments, according to the USDA -- and FSIS administrator Donald L. Houston decided he'd seen enough. Earlier this spring he announced that new requirements weren't needed and the proposal was withdrawn.

Well, apparently that was only the beginning. The milk producers hit the roof. Some farm groups, such as the National Grange, were furious and urged farmers to bombard the Agriculture Department with mail. The Grange was miffed because USDA cited Pillsbury's argument that substitute cheese had "no difference in nutrition."

And there was more. Rep. John D. Dingell (D-Mich.), chairman of the House Interstate and Foreign Commerce Committee, asked the General Accounting Office to investigate the department's withdrawal of the proposed rule. A House Agriculture subcommittee on dairy, livestock and poultry is expected to get into the issue at a hearing later this month.

And, most likely, there will be still more. James Barr, chief executive of the milk federation, said yesterday that his group is talking with some members of Congress about the possibility of dealing with the pizza-labeling issue as a part of the forthcoming budget reconciliation process.

"We'll focus on the budget avenue for now and see how it goes," Barr said. "It is a budget issue because we calculate that the use of substitutes has about a $50 million impact on federal spending."

The federation calculates that impact by figuring the amount of money Uncle Sam must spend to acquire American dairy surpluses that otherwise might be sold for use on pizzas were it not for the readily available casein imports that go into imitation mozzarella.

"We were quite surprised the department did that," said another federation spokesperson. "It is quite surprising that a federal agency would support a proposal that deceives consumers."

But wait just a minute, there. Ellen Haas, the director of Public Voice for Food and Health Policy, which is in constant guerrilla combat with the dairymen, took immense umbrage at the idea of the federation taking up consumer cudgels.

"We supported withdrawal of the proposed regulation," she said. "We were never on the side of the dairy interests. . . . It's really one industry against another -- the dairy industry against the casein users."