No one is going to accuse Rep. Henry Hyde, R-Ill., of patience when it comes to the Ocean Shipping Reform Act.

On May 5, just four days after the law took effect, Hyde convened a hearing on one of its key points, antitrust immunity for ocean carriers. He said afterward that he had ''no legislative plans to rush ahead'' with action.This week, however, fewer than six months later, Hyde - the chairman of the House Judiciary Committee - announced that he's taking very decisive action, and taking it quickly. He said he was introducing a bill to take away that antitrust immunity.

Supporters of the reform act are sure to argue that the powerful Republican is acting hastily, that he is giving short shrift to a large portion of the shipping scene in his analysis, and that OSRA is in fact working for much of the industry. They're right.

But they're obviously not going to deter Hyde from the course on which he has embarked. So they had better be ready and able to explain effectively why shipping lines today should be exempt from the nation's antitrust laws. And they had better be able to answer the question about immunity that he's making his battle cry: Who benefits?

Immunity from U.S. antitrust laws was extended to ocean carriers by the Shipping Act of 1916, a legislative effort to promote the American merchant marine. The idea was to enable American lines to participate in the system the rest of the world used. Since 1875, shipping lines worldwide have joined together in conferences to set rates on the trade lanes they served in an effort to prevent vicious price wars and maintain stable service.

Somehow it seems carriers have never been able to use immunity to their consistent benefit, at least in recent years; consultant Paul F. Richardson estimates their average revenue per load on major trade lanes dropped 25 percent from 1985 to 1998. But shippers despise it nonetheless, and understandably. After all, what customer likes his suppliers banding together to fix prices, even if they're not very good at it?

Carriers' antitrust immunity in the United States exists today not because shippers like it, but because there would have been no shipping-reform law without it. It was part of the shipper-carrier quid pro quo that ended four years of tangled debate, resulted in wide industry support and enabled OSRA to win passage in Congress last autumn.

The law allows shippers and carriers to negotiate confidential, one-on-one contracts that meet the individual needs of each, effectively bypassing the conference system. But carriers are still allowed to meet in discussion groups and promulgate voluntary rate and service guidelines for themselves.

OSRA was a compromise. It was a reform measure, not a full deregulation action. But it has certainly succeeded in boosting competition and shaking up the shipping marketplace.

It's dealt a body blow to conferences. Some have disbanded outright. Others are all but irrelevant. Carrier discussion groups certainly exist. But it's the market, not the groups, that appears to be driving the action. In the cargo-rich trans-Pacific eastbound trade, for example, confidential contracting has been robust and lines' rates have softened as additional capacity has been added.

Even Rep. Hyde acknowledged this week that ''there is no doubt that the OSRA creates a better world that that which went before.''

But the longtime lawmaker is sensitive to the concerns of a segment of the industry that has been unhappy with shipping reform from the beginning. Non-vessel-operating common carriers, who handle the cargo of smaller shippers, are not allowed by the new law to offer confidential contracts to their customers, as vessel-operating carriers can. And small shippers themselves feel weak and at the carriers' mercy in a system in which rates are no longer public.

Hyde - whose sensitivity no doubt is heightened by a concentration of NVOs in his congressional district - cited several reasons for his move against antitrust immunity. He said there were continued complaints about abuses by discussion groups, and that U.S. trading partners have begun to move away from antitrust immunity.

The former may be true, but the biggest volume of complaints, about the inbound Asian trade, came just before OSRA took effect. The latter statement is a bit oversimplified; while there is growing global questioning of immunity, it remains in place.

But Hyde was on the mark in noting the biggest change since OSRA came into effect: the agreement to sell U.S.-flag Sea-Land's international services to Danish-based Maersk Line. When that pact goes through, perhaps next month, there will be no major U.S.-owned global shipping lines left.

The purchase will be structured to allow Sea-Land's ships - like those of APL Ltd., the next most-recent U.S.-owned line to be sold to a foreign concern - to continue flying the American flag. But the company itself will no longer be American.

That will undercut philosophical support for antitrust immunity as it will for many other features of the U.S. maritime industry. It will open the way to intensified criticism of immunity as a special benefit for foreign lines. At the same time, it is also likely to undercut active support by diminishing the presence of American shipping companies in Washington.

Meanwhile, the National Industrial Transportation League, which represents big shippers, is working hard to walk a fine line between keeping its OSRA bargain and joining with shipper groups abroad in support of laws that oppose price-fixing by conferences.

And Hyde has gone out of his way to sidestep opposition by organized labor, which was against confidential contracting for NVOs, by retaining immunity for labor-management agreements on benefit-assessments, as well as for ports.

Well-timed or not, a reappraisal of the justification for antitrust immunity for ocean carriers is about to begin. Hyde himself estimates it could take ''a couple of years.'' It will be an interesting period.