IT HAS BEEN an exciting month for new antitrust theories down at the Federal Trade Commission. First, word leaked out that the staff has recommended a case be brought against the four companies that make lead additives for gasoline. Then, an administrative law judge announced that a new trial will not be necessary in the cereal case; he will get the evidence he needs by reading the 36,000-page transcript of testimony heard by his predecessor, who retired last fall.

Both events sound plausible enough - unless you think about them. If you do, you have to wonder. . . Why is the FTC trotting out a novel theory of law against a dying industry? Why is the cereal case, which began seven years ago, still incomplete?

In both cases, the FTC's staff is trying to win approval for legal theories that would expand the reach of the antitrust law. None of the companies is charged with conspiring to fix prices or allocate markets-the classic antitrust violations. Instead, the staff is attacking the cumulative effects of business practices and market strategies the companies have individually adopted and the structure of the two industries in which the number of big competitors is limited.

It may be that the FTC's legal theories are sound. The antitrust laws are so imprecise and corporations are so adept at skating around their edges that regulators constantly seek new ways of getting at new, or even old, problems.

But something is wrong when it takes more than seven years to lay the factual basis that the courts will need to sustain the theory brought out for the first time in the cereal case. The cost to the government and to the four cereal companies of the seven years of litigation already consumed - and there are at least five more years before the courts reach a final decision - is astronomical. There has to be a better way to establish new antitrust theories than that. And there is: new legislation.

Something quite different is wrong when the target of the other novel theory is an industry that is in sharp decline. Even if the FTC wins this case against the makers of lead additives, the benefit to the public will not have been worth it. The life expectancy of the case is probably greater than the life expectancy of a majority of the plants making the additives. Surely the government has a better use for tax dollars than spending them to pursue an industry that is being driven out of business by government policy and that never had much of an impact on consumer prices anyway.