He drives three motorcycles and a sports car, wears a gold chain around his neck, and answers to the nickname "Black Prince."

He is negotiating a treaty he once helped draft, then hired on as an industry lobbyist to help kill, and now is back in government trying to revive.

Your good gray diplomat, he is not.

Yet Leigh S. Ratiner finds himself center stage in one of the most sensitive, vexingly complex diplomatic adventures ever undertaken--the United Nations Conference on the Law of the Sea.

The 158-nation powwow has been going on in near-obscurity for eight years. It is scheduled to end next week, although such deadlines have proved rubbery in the past. Either it will have 158 signatories or 157.

The potential holdout is the United States, which has been locked in a struggle with the Third World over such issues as: Who owns the floors of the oceans? Who will mine the mineral riches that lie there? Who gets the profits?

Ratiner, as deputy chairman and de facto chief negotiator of the U.S. delegation, is the functionary in the middle of the argument. His enemies from the deep sea mining industry, and they are legion, insist he is something more--a cunning, unpredictable pragmatist all too ready to barter away their freedom of the high seas.

Under the pressure of an April 30 deadline, negotiations have suddenly heated up. Ratiner, bargaining within a small working group of the conference, this week devised a tentative allocation formula that for the first time provides guaranteed ocean bed mining sites to the five multinational mining consortia--dominated by such U.S. companies as Kennecott Copper, United States Steel, Standard Oil of Indiana and Lockheed Aircraft--that have sunk nearly a half-billion dollars into deep sea extraction technology but have yet to embark on any commercial ventures.

"It was a feat of unfathomable proportions," the Brooklyn-born Ratiner, with characteristic immodesty, said of the breakthrough.

But many more hard issues remain, and Ratiner is operating on a slippery slope. The very day he reached the tentative accord, an industry delegation was meeting with White House counselor Edwin Meese III. The group reportedly voiced strong objections to the treaty, and some suggested that the United States should pull out of the talks immediately.

"They're worried that I just might be able to come up with enough clever fixes to make the thing fly," Ratiner said.

Such turbulence on the home front is nothing new. Ratiner's appointment to the delegation last month provoked a furor within the tight little community that has followed every thrust and parry of the negotiations. But it was a furor with a twist: the competing ideological camps almost seemed to have exchanged uniforms.

The deep sea industry Ratiner once represented began scheming to have his appointment scuttled (they succeeded in denying him the personal rank of ambassador he had sought but haven't yet gotten him off the delegation); the globalists who long had viewed him as an adversary sprang warily to his defense.

The flip-flop bespeaks a shift in the balance of power among the competing U.S. camps. The forces of free enterprise were once delighted to have the pragmatic Ratiner in their stable. But now, with their orthodoxy back in favor and a friendly administration in power, he fails their ideological litmus test.

The pro-treaty internationalists, who had little use for Ratiner when their notions of income redistribution were setting the agenda for the conference a decade ago, are now welcoming him back as the one man who might salvage the treaty.

The conference has covered a broad range of ocean issues, but the most controversial provision was crafted in 1975-76 by Henry A. Kissinger, then secretary of state. It called for a parallel system of deep seabed mining in which an international authority, dominated by the Third World, and the private mining consortia of the developed nations would share access to the treasure-trove of manganese, cobalt, nickel and copper on the ocean floors.

By design, the arrangement was heavily stacked in favor of the international authority. Private companies, for example, would have to do all of the costly exploration and then turn over their most promising sites to the authority's own mining concern, the "enterprise." The United States and other industrialized nations would provide start-up costs and transfer technology to get the enterprise going. The profits would go to the Third World countries.

It was all designed to redistribute wealth from North to South. The developed nations went along because, under the thinking that prevailed in the 1970s, it was a price worth paying for other provisions of the treaty, in particular those that fixed territorial boundaries and guaranteed free peacetime navigation through all straits and channels.

During that period--what some in industry refer to as the "years of Santa Claus"--Ratiner was admired by industry as a hard-liner, a man who felt this country was giving away too much.

When he left government in 1977 after working on the Law of the Sea issues for four different government agencies over the span of a dozen years, he was hired by Kennecott Copper to lobby against the treaty.

Then the Reagan administration came to power, brimming with many of the concerns about the treaty that industry had harbored from the start. The new administration put the negotiating process on hold for a year while it reviewed all that had transpired.

As part of the review, the administration hired Ratiner--who had since been let go by Kennecott in a personality clash with management--as a consultant to study whether private companies would be able to borrow money to mine the oceans if they lacked clear title to sites as a result of a refusal by the United States to sign a treaty. Ratiner concluded there could be serious problems on that front.

Armed with that and other information, the administration decided in January to return to the bargaining table but to insist on major changes in the treaty.

That decision in itself troubled some of the ideological purists within the industry and their growing cadre of allies in Congress. They had come to believe that no good could come of a treaty negotiated in a 158-nation forum dominated by the Soviet bloc and Third World interests; that the United States would be better off divvying up the floor of the ocean in a mini-treaty with a half-dozen or so developed nations, the only ones that actually had the resources and technology to harvest minerals from the oceans.

Ratiner became a lightning rod for their concerns. "There doesn't seem to be a whole lot of principle there," Rep. Jack Fields (R-Tex.), a leading industry voice in Congress, said of Ratiner. "You just never know where he is coming from."

Fields went on to suggest that Ratiner's "experience and background would carry a premium" were he to set himself up as a private lawyer representing clients before an international authority dominated by Third World diplomats, and speculated that his negotiating posture might be skewed by such mercenary considerations.

Not everyone takes quite so cynical a view of Ratiner, but he has managed, with his hard-edged personality, calculating mind and outsized ego, to collect more than his share of enemies. "Leigh can be a devastating negotiator--and I mean that in all its ramifications," said Rep. John B. Breaux (D-La.), an expert on the law of the sea and a man Ratiner considers an ally. "He can be particularly effective as a gut in-fighter, and he can be particularly offensive as well. I'll say this: he's not a yes man. He's not going to bow to every constituent interest."

That is precisely what worries industry. "He is a brilliant man who has all the contacts and knows all the nuances of the treaty, but someone's got to manage him," said one industry spokesman, who asked not to be identified. "I'm afraid delegation chief James L. Malone just isn't up to it."

Ratiner professes to be troubled by all the industry backbiting, and sources say he has threatened to quit several times in the past few weeks, but a chunk of him obviously revels in the action.

"Everyone seems to be afraid that I'm going to cut some sort of deal in the kitchen of my apartment with my buddies," he said one recent weekend, wearing faded blue jeans and loafers and chain-smoking Dunhills in a spartan office in the U.S. mission across the street from the United Nations.

"They've always considered me too quick, too running ahead, too clever by half," he continued. "Even when I worked for industry, they used to hire other people to check up on me. They were always convinced I had something up my sleeve."

To the criticism that he doesn't do things the way normal diplomats do, meaning putting together elaborate position papers and getting detailed clearances from all the layers of bureaucrats, Ratiner says:

"Negotiation is an art, and I am an artist. I don't always knows where my next brush stroke will fall."

Then he tries out another metaphor: "It's like being in a boxing ring. You watch the other guy's moves. Is he scared of you or not? Have you successfully hyped your reputation? You feel your way into it, jabbing, provoking, trying to draw out a reaction."

And then: "People say I have a Byzantine negotiating style, but consider the forum I work in"--gesturing toward the U.N. building--"Byzantium."

And finally: "I enjoy the theater of it. The Third Worlders go in for that approach. The western diplomats don't."

Ratiner insists that he remains scrupulously faithful to his instructions from the White House, and that, whatever controversies may swirl about him outside the U.N. building, "I have complete credibility with the other negotiators.

"My problem is not with the front. It's with the flanks."