Many people are passing up a fine retirement nest egg in IRAs

The IRA may have shifted billions of dollars into the retirement-savings system, but that money seems to be coming from a relatively small portion of the population.

Even though individual retirement accounts have been available to all working people for more than two years, even though studies show that most people under 40 expect to receive little if any social security benefits when they retire, and even though the public has been bombarded with advertisements and publicity about it, IRA participation remains relatively low. Only about 25 percent of those eligible have IRAs, according to Wesley Howard, editor of the IRA Reporter. Some 40 percent of those eligible participate in a similar account in Canada.

Although participation among people earning $30,000 and more is higher than 25 percent, many people who could be taking the first small steps toward more financial security at retirement are not doing it.

Part of the reason, Mr. Howard says, is that there is still a ''lot of confusion out there'' about the basic rules of the IRA business. He recalled a recent survey that found 70 to 75 percent of those responding still believed they could not open an IRA unless they had at least $2,000. That is the maximum an individual can put in; an IRA can be started at a many banks for one dollar.

Many people also believe they cannot touch the IRA money before retirement for any reason. Truth is, you can take it out anytime you like if you're willing to absorb the 10 percent penalty and pay taxes on the withdrawal. After several years, this negative can be nullified by the benefit of tax-free compounding of interest.

Then there is confusion about the IRA alternatives - a condition that has only become worse in the last two years. At the beginning of 1982, when the ''universal'' IRA became available, the choices could be narrowed down to a certificate of deposit at a bank, a mutual fund, and perhaps stocks and bonds invested through a self-directed brokerage account.

Today banks and savings-and-loans offer such a wide variety of maturities and interest rates that it is possible to choose from more than 1,000 CD combinations at some institutions. Mutual funds are continually inventing new products designed specifically for the IRA, and brokers are coming up with products of their own that can be used for IRAs. They include variations on the zero-coupon bond with catchy names like TIGRS, CATS, and LIONS.

For the more daring individual, there are also limited partnerships that will put your retirement kitty into real estate and oil and gas investments.

One way to narrow down the list of alternatives is to examine carefully your own tolerance for risk. Many people believe the IRA will be their only source of income at retirement, so they are reluctant to take any risks with it. Thus, they stick to banks and thrifts where they get $100,000 of federal deposit insurance, but they know up front - for a few months, at least - what the yield will be.

Yet for many people, the IRA may be exactly the right vehicle for taking a few chances. The savings and investment climate has changed dramatically several times in the last few years. Someone in his 30s or 40s, then, with two or three decades before retirement still has time to change directions in the investment road. Thus, if you decided to use your IRA to play the stock market but lost money in your self-directed brokerage account, you have plenty of time to start over in a more conservative investment, if you like.

Even older workers, if they already have a sufficient retirement income in place, such as a pension, combined with savings, home equity, and investments, may want to put more aggressive investments in their IRAs. There's a catch to this, however: If you invest in stocks outside an IRA, for instance, and hold them more than a year, the gains are taxed at the lower capital-gains rate, an effective maximum of 20 percent. This advantage is not available in an IRA, where withdrawals at retirement are taxed as ordinary income. Also, capital losses cannot be used to offset gains, as with stocks other than in IRAs.

There are two reasons for considering IRAs now. First, if you have not yet filed your tax return for 1983, you can still open an IRA and take the deduction off that year. If you already have an IRA, you can make a 1983 contribution even after your return is filed - as long as it is made before April 16. Thus, some people who have filed early are planning to use their refund for their 1983 IRA deposit.

As for the 1984 contribution, the sooner it is made, the more time it will have to earn tax-free interest. Next week: Choosing among some IRA alternatives