Jittery U.S. home computer companies fearing competition and curious consumers need no longer ask, "Where are the Japanese?"

They've arrived.

After nearly a year's delay, more than a dozen Japanese companies at this week's Consumer Electronics Show premiered the first wave of their low-cost home computer hardware and software built around the MSX standard.

The MSX approach, which was formally announced in Tokyo more than 18 months ago, is essentially Japan's bid to pierce the turbulent but potentially lucrative U.S. home computer market. The MSX is a standard for the home computer just as VHS and Betamax are standards for video casette recorders.

The standard links microprocessor, sound and video chips into a unified system, which means that computer programs written for one MSX computer can run on all MSX computers. In effect, the MSX standard turns a home computer into a generic item.

Japanese consumer electronics companies have generally been very successful when manufacturing generic products such as video cassette recorders and color television sets based on set standards.

"This is MSX's first week in the United States," said Kay Nishi, vice president of technology for Microsoft, who has aggressively championed MSX as a global home computer standard. "Within two years, we will see if it is successful."

More than 15 of Japan's largest consumer electronics companies, including Matsushita (Panasonic), Hitachi, Sony, Toshiba and Yamaha, have endorsed the MSX standard and are already shipping machines based on it in Japan and Europe. N.V. Phillips, the giant Dutch electronics company, is also selling MSX machines. Prices generally range from $99 to $599.

Microsoft's Nishi, who prefers to call MSX a "concept" rather than a standard, estimates that last year nearly half a million MSX machines were sold in Japan while roughly 100,00 were sold throughout Europe. Industry analysts generally agree that MSX is doing well outside the United States.

Nishi projects that "somewhere between 250,000 and 500,000" MSX home computers will be sold in the United States this year after the machines begin hitting the retail shelves in August.

Others are skeptical of the Japanese invasion and MSX's prospects for success in America.

"I don't think they have a prayer," said Tim Bajarin, a microcomputer industry analyst with Creative Strategies International in California.

Spokesman for Atari Co. and Commodore International, the two leading U.S. home computer companies, dismiss MSX as a serious threat. Software companies are reluctant to write MSX programs.

"It's too little, too late," said Michael Reichman, director of Product Development for Batteries Included, a leading home computer software company. "They'll have a tough time being competitive even at the $100 price point. Besides, MSX is really a games-oriented approach and consumers are now moving away from that."

Nishi and other MSX supporters recognize these shifts and are seeking to emphasize other non-home computer aspects of MSX to gain greater market acceptance.

"MSX is also an interfacing standard," said Gilman Louie, president of Nexa Corp., an MSX software company, arguing that MSX may become a communications standard for an "intelligent appliances network" that links household goods such as video cassette recorders, telephones and microwave ovens to MSX controllers. Louis postulates that it would be possible to call up a VCR on the telephone to tell it to record a program using MSX.

The Japanese MSX companies, most of which are also appliance manufacturers, already are integrating MSX into some of their products. General, a major Japanese appliance company, has designed a color televison monitor with MSX capability; Yamaha manufactures an electronic music keyboard that is MSX compatible; and Sony's MSX machine can superimpose printed titles on video cassette tape for playback on the VCR.

"Initially, MSX will be an accessory to an audio-video system as a controlling device," said Harriet Fox, a Microsoft consultant. "Ultimately, it will be used as a personal productivity tool much like the Apple and IBM."

It is still too early to tell whether the MSX machine and the "intelligent appliance" concept will take hold in the United States as it has in Japan. Response to the MSX booth at the electronics show was cool.

Nishi is not worried. "My mission is to prove that MSX is signficiant in the long run. This is a long-term business," he said.

"I don't know if MSX will succeed here," says one buyer for a major retail chain who asked not to be identified, "but I'm going to be watching this very carefully."