When the Justice Dept.'s new antitrust chief, Joel I. Klein, approved the $23 billion merger of Bell Atlantic Corp. and Nynex Corp. on Apr. 24, cheerleaders for competition were appalled. "The only thing Justice would use its enforcement authority on is the merger of Jupiter with Neptune," scoffed former New York State antitrust official Lloyd Constantine, one of many critics who believe Clinton Administration trustbusters are asleep at the switch.

Potshots such as these should buoy Corporate America. For seven months, Klein, former deputy White House counsel, has kept a low profile while serving as Acting Assistant Attorney General for Antitrust. But with the Senate expected to confirm him before Memorial Day, the scholarly Harvard Law School grad and appellate lawyer is finally making his mark. And the early betting in legal circles is that Klein, who declined to be interviewed, will be a lot less aggressive than his predecessor, Anne K. Bingaman.

While Bingaman forced Microsoft Corp. to change some of its marketing practices, Klein, 50, is reticent to intercede in rapidly evolving high-tech industries. So far, he's relying on fast-paced innovation to assure competition. He O.K.'d the Bell Atlantic-Nynex deal, one of the largest U.S. telecom mergers, because he expects that new local-service providers--including wireless and long-distance companies--will eventually compete for business.

FOOT-DRAGGING. Klein is also unlikely to step up Justice's investigation into the bundling of Microsoft's Internet Explorer browser with its dominant Windows operating system. The reason: Rival Netscape Communications Corp. is way ahead with its Navigator software and has grabbed 75% of the browser market without any help from Uncle Sam. With Klein dragging his feet on the case, state trustbusters in Texas and Massachusetts have launched their own inquiries.

Klein is more inclined to make an antitrust splash in mature markets, where it may be easier to build a strong case under current antitrust law. "He's willing to push the envelope, not to burst it," says Richard L. Rosen, a former Justice Dept. attorney who worked on Bingaman's Microsoft case.

For example, Klein is probing Visa USA Inc. and MasterCard International Inc. The two nonprofit bank associations, with 75% of the market, bar member banks from issuing rival cards such as American Express and Discover. Justice is weighing a case that may restructure the industry by keeping banks from being owners of both Visa and MasterCard simultaneously. "This may be the most important case of his tenure," ventures former Bingaman aide Robert E. Litan of the Brookings Institution.

NEW TUNE. Another likely target: the nation's music-licensing duopoly, the American Society of Composers, Authors & Publishers and Broadcast Music Inc. Klein may abolish or rewrite 50-year-old antitrust rules governing music licensing. Restaurants, cable programmers, and radio and TV broadcasters must now buy expensive blanket licenses of the music publishers' full repertoire--regardless of whether they want the whole thing.

Klein has also shown some zeal for exploring antitrust territory abroad. He wants foreign counterparts to help go after offshore companies that harm U.S. consumers. That's why Klein asked the European Union on Apr. 28 to investigate possible anticompetitive actions by European airlines to block the use of American Airlines Inc.'s Sabre computer-reservation system.

Closer to home, Klein is content to pick his shots carefully. He hopes to concentrate efforts on a few landmark cases rather than a string of marginal suits. But first, he'll have to overcome his reputation as the too-cautious trustbuster.

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