Over one million Americans may think their most intimate business and personal phone conversations are private.

They aren't.Words spoken over cellular telephones - the wireless models most often seen in automobiles - can be simply plucked from the airwaves, grist for eavesdroppers.

All that's needed is a radio scanner - inexpensive, easy to operate and hated by cellular phone makers, who fear many callers would shun the mobile units if they knew of scanners.

The conflicting interests of the telephone and scanner industries have sparked a battle pitting technology against technology, the right to privacy against the right to airwaves. Burgeoning profits are at stake.

A listener getting acquainted for the first time with a popular scanner - a squat, black box resembling a radio that can also monitor police, fire and citizen band frequencies - recently reported overhearing some telling cellular phone conversations.

"We're getting $1,000 back on our federal taxes, $500 from the state," a man said to a woman. Her audible elation quickly gave way to disappointment. "We're paying off all our debts first. The deck's on hold," he said.

Many calls involved business. A lawyer confided that an agricultural processing plant was going to be hit with a multimillion-dollar federal fine. Another asked whether a federal judge would interfere with a company's decision to buy a mutual fund.

Some calls offered "more real life intrigue than most soap operas," as an Indiana scanner maker promised in a 1985 ad. One sounded like a couple of drug smugglers in action with vague talk of "snow" and secret payments.

About ten million scanners have been sold, and an estimated one million Americans own models that can monitor cellular frequencies, between 800 and 900 megahertz.

They may know more about their neighbors than about the law. Congress in 1986 prohibited unauthorized, intentional monitoring of cellular phone conversations. The offense is a felony, punishable by a $500 fine.

But the scanner industry claims cellular phones use airwaves that have always been open to public listening. What is more, they contend, the law can't be enforced and has been used to mislead cellular phone users.

"The law hasn't made cellular phone calls any more private," said Robert Hanson, head of the Scanner Association of North America in Western Springs, Ill.

"It has done a disservice and provided a smokescreen by saying it's illegal to listen in. They (cellular manufacturers) are trying to advertise a sense of security," he said.

To date, no one has been prosecuted under the law, in effect since January 1987. Republican Rep. Michael DeWine of Ohio said while the law was being drafted that the Justice Department had told him it had "absolutely no intention of enforcing the bill."

Justice Department spokesman John Russell said the government had told Congress the law would be "very, very difficult" to enforce because a prosecutor would have to prove intent to eavesdrop. "Who's to know?" Russell asked.

Robyn Nietert, a telecommunications law expert with the law firm of Brown, Finn & Nietert who represents a large scanner manufacturer, said phone makers believed the law would head off any congressional attempts to require them take costly steps to modify their products to assure privacy.

But, Nietert said, cellular phone makers overstate the costs of adding devices that scramble or encrypt cellular conversations. "Every European system is secure. If our phones had been built secure from the beginning, it would have cost only another 15 percent," she said.

At least two major manufacturers, Tandy and Uniden, have stopped selling scanners with cellular frequencies. But experts say scanners can be easily converted to receive such calls.

The Federal Communications Commission is considering a proposal to require scanners to bear a label, warning that it's illegal to eavesdrop on cellular calls.

Ironically, some cellular manufacturers oppose the labeling proposal.

Robert Maher, president of the Cellular Telecommunications Industry Association, said scanner makers were trying to avoid legal problems by transferring to the public the burden of complying with the law against evesdropping.

"I am disturbed that people are trying to circumvent this (law)," Maher said.

But scanner defenders say that if the cellular phone industry is so disturbed, it should equip phones with devices that scramble conversations, or put warning labels on phones.

California already requires such labels, but the FCC last year turned back a proposal to force all phones to carry them.

"That's like putting a label on a person that says, 'Do not mug, shoot, rob, rape or pillage,"' said Maher. "We don't think the responsibility should be totally on the individual to protect his privacy."

Meanwhile, Robert Horvitz of the Association of North American Radio Clubs warns that cellular phone calls continue to be picked up by millions of Americans, including anyone with a television offering UHF channels 80 through 83.

"There is no way to confiscate millions of TVs," he said.