In the late 1980s and early 1990s, antitrust was widely seen as an antiquated topic, but things have changed dramatically in the last few years. Antitrust seems to be entering a new golden age.

The antitrust action against Microsoft is only the most visible sign of the revived interest. Europe's scrutiny of mergers and proposed mergers, including Boeing-McDonnell Douglas and MCI WorldCom-Sprint, indicate that the new trend is not confined to the United States.One of the realities of globalization, however, is that markets are no longer constrained by national borders. The most important challenge facing U.S. antitrust advocates is adapting to the global economy a body of law that was designed primarily for dealing with a national economy.

Clearly, the realities of international competition have forced a fresh look at allowing increased collaboration between national companies to meet international competitive challenges. Sematech - the research consortium among U.S. semiconductor companies (and originally the U.S. government) - has demonstrated that there are important potential benefits to collaboration that should not be restricted by antitrust enforcement.

That said, it's undeniable that the country as a whole benefits from a diverse marketplace in which companies compete for customers on the basis of price and innovation.

Perhaps one of the most valuable things to be learned from the Microsoft case is that antitrust policy does seem to have a role to play in the ''new economy.''

The final resolution of the Microsoft case could easily be years away, and Microsoft certainly deserves the right to pursue all legal avenues reasonably available to it to defend itself.

Already, however, there are important lessons to be drawn from the litigation. Microsoft and its chairman, Bill Gates, have argued stridently that the old concepts of antitrust cannot be applied to the new economy of software and microchips.

This argument was initially appealing to many, including me. But it is striking how little Microsoft was able to produce to support this position. In fact, as the case unfolded it seemed that the parallels between Microsoft's actions and those of past trusts were too strong to be ignored.

Even more surprisingly, as the case enters its next phase it seems that many of the remedies employed in the past - court-ordered breakups and limits on commercial behavior - are, in fact, potentially viable in the new economy, just as they were in the old economy.

So antitrust is likely not as antiquated as it once appeared. Still, however, there are competitive realities that must be addressed.

Consider the recent merger of Boeing and the other remaining U.S. airframe manufacturer, McDonnell Douglas, which ultimately was approved by U.S. authorities.

Even though Boeing would still face intense international competition from Europe's Airbus, many were concerned that this merger would lead to troubling concentration in the production of large airliners.

Airbus, however, would not exist were it not for generous subsidies by European governments. Since the United States is not inclined to grant similar subsidies to Boeing, might it be appropriate to loosen antitrust scrutiny here? And in cases of other U.S. companies forced to compete with foreign governments as well as companies?

Beyond that, Europe's increasing interest in mergers involving major U.S. companies, like Boeing and MCI-WorldCom, raises other questions.

Europe has every right to promote a competitive marketplace in order to ensure low prices for its consumers. There is, however, a fine line between this type of legitimate action and measures designed to promote the interests of the European competitors of the U.S. targets of the EU actions. The latter, of course, is merely another type of protectionism.

Since so many of the world's largest companies - General Electric, General Motors and Microsoft - are U.S. companies, the United States must remain on guard to ensure that a global wave of antitrust actions does not become an excuse for a wave of anti-U.S. protectionism.

The day may not be far away when the United States must decide whether an antitrust action against Microsoft in Europe, Japan or elsewhere is a legitimate effort to follow a U.S. example, or merely an effort to boost a foreign Microsoft rival.

At some point, these issues will require an international agreement. Negotiations have already been proposed on an international antitrust agreement, but from a U.S. perspective they are likely to be long and difficult.

A diverse, competitive marketplace still remains very much in the best interest of the United States. The new competitive realities of the global market, however, make that goal harder to pursue than ever before.

An unfortunate truth, it seems to me, is that most other countries are more interested in building up national champions - and perhaps tearing down U.S. national champions - than building a competitive marketplace.