A MARRIAGE of Du Pont and Conoco would produce a company that would be at once the largest chemical producer would be at once the largest chemical producer in the country, and the second largest coal producer. It would be the ninth largest oil company, and a leader as well in biological and medical technology. Would that violate the antitrust laws? Ten years ago, the answer would have been almost automatically yes. Today, it may be otherwise. For those products, it's no longer the national market that's the proper measure, but rather the world. On a world scale the trade in chemicals, oil and the rest is pretty competitive, and not even Super Du Pont could really be called dominant.

But this kind of conglomeration, generally speaking, has not often proved very profitable or efficient. Du Pont's management has taken a rather daring -- and certain expensive -- step to rescue Conoco from the foreigners. Not everyone would consider Canadians to be the most menacing of foreign influences, but Conoco has been vociferously protesting the Canadians' approach to the tune of "The Start and Stripes Forever."

Oil is recurrent theme in the current wave of corporate mergers. The enormous increses in oil prices, two years ago, have left most of the oil producers load with cash. That has made them very attractive targets for takeovers unless, of course, they spend their money quickly by taking over someone else. In this case a Canadian distilling company, Seagram, sold its oil and gas operation a year ago for more than $2 billion, and since then has been looking for ways to invest the money. When it began to move toward Conoco, Conoco hastily invited a friendly takeover by Du Pont.

That's fair enough. But, as part of its defensive campaign, Conoco has been trying to stir Congress into a reaction agains the Canadian invasion. There is, in fact, rising lfriction between Canada and the United States over oil. Most of the Canadian oil and gas industry is owned by American, and several American oil companies charge that the Canadian government's current energy policy is a strategy to force them to sell their Canadian properties, at cut-rate prices, to Canadians. That's a proper concern for Congress. But it's not directly related to the case of Conoco, which simply wants to escapt Seagram's embrace.

When foreigners want to put money into this country it ought to be encouraged, particularly when the investor comes from a country with a legal trandition and an open market very similar to this country's. Restricting foreign investment is a form of protectionism. It's reasonable for very big companies to defend themselves from anti-trust suits by arguing that they operate in worldwide competition. But those are the last companies in the world to take refuge from takeovers in protectionist politics.