The Justice Department's top antitrust lawyer detailed for the first time yesterday Reagan administration plans to allow high technology companies to cooperate on research and development efforts in order to compete better in world markets.

Assistant Attorney General William Baxter said if the companies that band together don't have a preponderate share of the world market their joint research efforts would be presumed to be lawful.

He told a National Association of Manufacturers meeting that ideally there would be room for between two and four similar joint R & D ventures in an industry. The combined size of the companies forming the research cooperative should be no more than one-sixth or one-fourth the size of all the other remaining firms outside the venture worldwide, he said..

"Rivalry is important in research," said Baxter, who takes an economic free market approach to antitrust enforcement.

The proposals are supported by U.S. companies that say that present antitrust law makes it illegal for them to band together for cooperative research, putting them at a disadvantage against foreign rivals that do cooperate. But Baxter said no administration bill has been sent yet to the Hill and he declined to speculate on its chances there.

As America's international trade picture has worsened, the concept has gained important allies, ranging from President Reagan to the National Academy of Sciences.

One group of computer firms, sparked by Control Data Corp., has already formed a cooperative R & D venture headed by retired admiral and former Central Intelligence Agency deputy director Bobby R. Inman. That venture, Microelectronics & Computer Technology Corp. (MCC), operates under a special letter of exemption from Baxter.

Despite his support for cooperative R & D ventures, Baxter cautioned the businessmen yesterday that this approach alone will not solve America's trade woes. He blamed trade deficits largely on an overvalued dollar caused by high interest rates, which in turn are caused by soaring federal budget deficits, he said.

It does not necessarily follow that less stringent antitrust laws in other countries have given foreign competitors a trade advantage over American firms, he added.

But American industry's international competitiveness would be aided by certain changes in the antitrust laws that make it hard to exploit and license intellectual property such as computer programs, he said.

"A very large number of very restrictive antitrust doctrines" have grown up in those areas, he said.

These changes include extension of patents on processes to include the products made by those processes. If this occurs it would make it harder for foreign competitors to sell goods in this country that resulted directly from the theft of a patented process, Baxter said. Another proposal would give patent, trademark and copyright holders greater control over their "intellectual property"--which in the field of high technology includes computer programs.