Here's something those IRA marketers don't want you to know.

People leaving their jobs with a bundle of company stock in their retirement plans can take advantage of little-known tax breaks - as long as they don't roll their stock into an individual retirement account.

But don't expect to hear about these breaks from the mutual-fund companies and brokerage firms that arrange IRAs. After all, if you put company stock into one of their IRAs, they collect account fees. If you hold onto a stock certificate, they get zip.

The amount of money at stake isn't trifling. More than 36% of the estimated $1.4 trillion in retirement savings plans, including 401(k)s, is invested in shares of employers' stock.

When you leave your company, you can take your company stock in several ways. You could cash it out, you could roll it into an IRA, or you could walk away with a stock certificate.

Cashing it out is a bad idea: you will owe income taxes on all the money, and maybe a 10% penalty. Transferring it to an IRA makes some sense, because you defer taxes.

But there's a third alternative, and it's a good idea for some people with a large amount of highly appreciated stock. Forget the IRA, and walk off with the stock certificate.

Sure, you will owe taxes - but only on the value of the shares at the time you purchased them, or whenever the company added them to your account. If your company's stock is a winner, this "cost basis" is usually much less than the stock's current selling price. For example, if the stock were selling for $50 a share, your basis might be just $10. You may also get a break on the 10% tax penalty on early withdrawals because it too applies only to your basis in the stock.

You can continue to defer taxes on all the share-price gains since the stock was initially purchased (the "net unrealized appreciation") until you sell the stock. You will, of course, pay annual income taxes on any dividends.

When you ultimately sell your shares, you will pay taxes on the appreciated value at long-term capital gains rates - usually 28% - rather than at income-tax rates of as much as 39.6% for wealthy people.

"The real sizzle is, I have tax-deferral and I legally have converted what might otherwise have been ordinary income into long-term capital gains," says Richard W. Friedman, vice president at Ayco Co., a financial planning firm in Albany, N.Y.

Unfortunately, no one mentioned any of the sizzle to Norbert Ramstack. When he left Detroit Edison Co. in 1986, he took his 630 shares to a Merrill Lynch broker, who advised him to put the stock into an IRA.

Had he kept the stock outside of the IRA, he would have paid income taxes immediately, but only on the cost basis of $9 a share, far below the market value of the stock. With the shares in an IRA, however, Mr. Ramstack will have to pay income taxes on withdrawals when the law says he must begin making them, at age 70-1/2.

His heirs will pay more taxes, too. If Mr. Ramstack dies with shares left in the account, they will owe income taxes on all the apprecia- tion in the stock since he left the company. Had he held the shares outside the IRA, they would have fared better. They would owe taxes only on the shareprice gain before Mr. Ramstack took the shares out of the plan, enjoying what's known as a "step-up" in basis.

The biggest potential tax benefit of owning employer stock comes to those who made after-tax contributions to a retirement plan in additional to saving the usual pretax amounts. Before or after retirement, you can usually withdraw after-tax contributions, for any reason - without paying income taxes.

Let's say your $100,000 401(k) balance includes $10,000 added by you, using after-tax dollars. You are entitled to take $10,000 in cash without paying taxes on it. But if you take company stock instead, you can withdraw shares whose "basis" is $10,000, even though its market value could be, say, $50,000.

"This is one of the big benefits of employer stock,", says Elizabeth Buchbinder, a partner at Ernest & Young. "No question, it makes owning the shares seem more attractive."

Most brokers and fund companies probably don't know much about this tax break. Nor do they have much incentive to tell you about it.

But employers usually have some information available to those who ask for it. To find out how much you might save in taxes by not rolling over shares into an IRA, ask your employee benefit depart- ment to provide you with the "net unrealized appreciation" (NUA) on those shares before you make any withdrawals.