WITH ONE MASTERFUL stroke, Rep. Dan Rostenkowski has propelled the House-Senate negotiations on the trade bill to an unexpectedly fast and promising start. He has proposed to the Senate to drop all of the most inflammatory and protectionist sections of the part of the legislation now under discussion.

As chairman of the House Ways and Means Committee, Mr. Rostenkowski has used his considerable authority to strip a remarkably large amount of the protectionism out of the bill as the talks begin. The Senate conferees are to meet this morning to decide how to respond -- whether to join him or to try to defend these attempts to protect specific industries and practices. Many senators have been saying that they want to produce a bill that strengthens American trade policy, not one that merely does favors for uncompetitive American producers. This will be an opportunity -- the first of many -- to show it.

Mr. Rostenkowski proposes to throw out the import quotas on lamb that the Senate wanted to impose while it simultaneously denounced Japan's quotas on American beef. He would throw out the language about requiring some imported autos to travel in American ships. He would throw out the complex provision on sugar tariff rebates that turns out to be a subsidy of hundreds of millions of dollars for four favored refiners. He would throw out the provisions for draconian and unfair penalties for customs law violations, relying instead on better customs administration. That's quite a list.

The conference has decided to start with the narrow issues. The harder and larger ones -- the Gephardt amendment and all the provisions on dumping and retaliation for foreign subsidies -- have been left until later. It is also true that in this first phase the worst items have less support in the House than in the Senate. Mr. Rostenkowski's list is only the beginning of a long process.

But it is a beginning that will influence the whole negotiation for the better. The chairman of the Senate Finance Committee, Lloyd Bentsen, has repeatedly said that he wants a bill to provide not protection, but a national trade policy that is clear, reliable and consistent. If the senators now work with Mr. Rostenkowski in the spirit of his proposals, Congress will be in a stronger position to call the Reagan administration to account for the shortcomings in its management of the current trade laws.