American Telephone & Telegraph Co. is in discussions with two large Japanese companies to enter the fast-growing market for laptop computers, according to industry sources. The machines would have something most existing models lack, however -- the ability to link up with other computers by radio.

AT&T is expected to announce a joint venture with trading company Marubeni Corp. to build the machines at an AT&T facility in the United States, using technology and various components from Matsushita Electric Industrial Co., a large Japanese electronics firm.

Details have not been worked out. But industry sources said AT&T expects to retain a majority ownership stake in the venture.

"AT&T and Marubeni have a research and development partnership and this partnership may result in some advanced product introductions in the future," said Herb Linnen, an AT&T spokesman. He declined to provide details. A Marubeni official declined comment.

Laptops, which allow people to harness major computing power in lightweight machines they can carry on the run, have turned into one of the fastest-growing products in the computer market. The Boston research firm the Yankee Group has estimated that 2.2 million units will be sold this year at a cost of about $2.8 billion.

The machines can communicate with other laptops, but generally only over phone lines. Industry sources said AT&T hopes to differentiate its machines from competitors' by using some kind of radio -- cellular telephone frequencies, perhaps -- to eliminate the need for laptops to be hooked up to telephones to transmit or receive data.

AT&T began selling computers after spinning off its local telephone companies in 1984 but lost money heavily in the highly competitive and fast-changing business. In the last year or two, however, it has turned that performance around with some major sales to the U.S. military and to American Airlines.

From the start, AT&T has attempted to stress not just the computers themselves, but the ability to tie them together into networks, using the company's long-distance telephone system. Its laptop would build further on that strategy.

AT&T would have important power to market such a machine, but it would not be the first company to offer such a product. For instance, Secure Technologies Inc., a small company based in Herndon, in 1988 introduced a machine that included a portable computer and cellular telephone.

The U.S. laptop market is already crowded with Japanese firms, which have considerable experience in electronics and miniaturization. The California research firm Dataquest Inc. ranks Toshiba Corp. and NEC Corp. No. 2 and 3 after Zenith Data Systems in the U.S. market for laptop computers. Both those Japanese companies manufacture machines in the United States.

Others supply entire or nearly complete machines that U.S. firms sell under their own names. A fast-selling Compaq Computer Corp. laptop, for instance, relies heavily on manufacturing by Japanese watch-maker and electronics company Citizen Watch.

Still, it is rare for U.S. and Japanese firms to leave majority control in U.S. hands when they team up to build computers in this country. In most cases in the electronics industry, said Susan MacKnight, chief economist at the Japan Economic Institute, a Washington think tank, "the Japanese operate as wholly owned subsidiaries -- either {they build} a new plant or they buy a company."

Matsushita, which had sales of $38 billion in the year that ended March 31, is known mainly as a consumer electronics company, selling under the Panasonic, Technics and Quasar labels. But it also makes personal computers and laptops and owns a majority interest in the Colorado company Solbourne Computer, which makes high-powered desktop machines known as workstations.

With annual sales of $110 billion, Marubeni is one of Japan's largest trading houses, dealing in commodities and manufactured products worldwide. It also has interests in manufacturing as well and a venture with Occidental Chemical Corp. to produce a specialty chemical in Texas.