America is gung ho about the euro.

After years of indifference and skepticism about the European Union's boldest project, U.S. businessmen and politicians are warming to the new currency due to be launched next January.Washington views the success of European economic and monetary union (EMU) ''as both very likely and very desirable,'' according to Vernon Weaver, the U.S. ambassador to the EU in Brussels.

Large U.S. companies, used to dealing with a single currency back home, often are ahead of their European rivals in preparing for the euro's arrival.

The euro also is also luring new U.S. investment to Europe. Monetary union is ''the most significant event in the history of bank notes,'' according to Currency Systems International, a U.S. note-processing firm that's moved to Europe. The potential market: replacing more than 12 billion national bank notes before mid-2002, when they will no longer be legal tender.

The United States has supported monetary union, although the euro eventually will challenge the dollar's role as a global reserve currency.

Monetary union has failed, however, to register big in Washington because no one could be certain whether the idea would take off. Recession and currency crises forced EU leaders to abandon the original 1997 launch date.

But the U.S. is finally paying attention as EU leaders prepare for a special summit in Brussels on May 1, to pick the countries that will participate in the euro's launch.

They are virtually certain to wave through 11 of the 15 EU members, including Italy, a borderline candidate because of its huge public debt. This has fanned fears in Europe, as well as the United States, that the euro will be a weak and unstable currency.

British Prime Minister Tony Blair will seek to allay U.S. fears when he meets with President Clinton today in Washington and later with Alan Greenspan, chairman of the Federal Reserve.

''It's essential there is a strong euro,'' Mr. Blair said. Italy will be judged ''in exactly the same way as other applicant countries.''

Britain currently holds the EU's rotating presidency, but Mr. Blair's position has been weakened by his decision not to enter the monetary union next January. He plans to put membership to a referendum when he judges the British and continental European economies are in synch - probably just before euro coins and banknotes are issued in 2002.

That's too late for a growing number of worried U.S. companies with British subsidiaries. Many are warning they may be forced to rethink their investment strategy, a worrying development for a country that grabs more than 40 percent of all foreign investment into the EU.

For the first time, many U.S. firms are thinking about opening plants in the ''euro zone'' that will cover continental Europe after 1999.

Agco, a tractor manufacturer, is the latest U.S firm to sound a warning, saying investment at its plant in Coventry, England, Europe's largest, will suffer if Britain stays aloof from the euro for a significant period.

This follows similar warnings by four U.S. companies with more than 10,000 employees at more than 30 British plants: TRW, a defense and auto parts firm; AMP, the world's leading maker of electric connectors; Case, another tractor manufacturer; and Ingersoll-Rand, an engineering group.

Siemens, the giant German electrical conglomerate, plans to make the euro its ''house currency'' in October. And many large U.S. multinationals are expected to do the same before the new currency becomes the official standard.