These days, a decent money market mutual fund pays a yield of about 61/2 percent.

So how does 22 percent sound?Thanks to income taxes saved every year, that's the minimum return you effectively get from an employer-sponsored retirement savings plan - a 401 if you work for a company, a 403 if you work for a non-profit organization, university, church or public school system.

"The minimum you can earn on a 401 in Massachusetts is 22 percent because of the tax savings," says Michael Hogan, executive vice president of Associates Benefit Planning Service Inc., a consulting firm in Framingham, Mass. Because the money goes into these plans on a pretax basis, he explained, it's equivalent to getting a 22 percent yield on an after-tax deposit into a bank account or money fund. For people in higher tax brackets, the numbers are even better.

This benefit is even more apparent now, in the middle of the tax-filing season: If you had put $100 a week into a 401 or a 403 in 1990, your taxable income is immediately reduced by $5,200. So a person earning $35,000 a year pays taxes on just $29,800, even less after taking all exemptions and deductions.

On top of that, many employers, especially larger ones, match each employee's contribution to a 401 or 403 The size of the match varies, but many employers kick in half the employee's deposit. In a survey taken last summer by Charles D. Spencer & Associates of Chicago, 82 percent of companies with 251 to 1,000 participants matched employees' 401 contributions; 86 percent of those with 1,000 to 5,000 participants matched.

This year employees can contribute up to $8,475, or 25 percent of their salary, whichever is less, to a 401 says Jonathan E. Flitter, vice president and general counsel at Pentad Corp., a benefits consulting firm in Waltham, Mass. People who have 403 plans can sock away up to $9,500, or 20 percent of salary, whichever is less.

If you are offered a 401 or 403 plan, then, regardless of whether other plans are offered, take maximum advantage of it. Not only will it cut your tax bill, but even a little money can build up. A 30-year-old who puts aside $1,500 a year - less than $30 a week - would have $258,475 at age 65, assuming 8 percent annual appreciation, which many stock funds are posting these days, Flitter figures. At 9 percent, the total comes to $323,566.

If you wait until you're 50 to start, $1,500 adds up to just $40,728 at age 65, with 8 percent interest, while 9 percent brings the total to $44,041.

However, benefit consultants and investment advisers say, while 401 and 403 plans hold great tax benefits, there could be major problems ahead for retirees. First, more companies are replacing traditional defined-benefit pensions with 401 , which are much less complicated and costly to administer. In theory this should be fine, Hogan says, because, given enough time, a worker can create a bigger pension with a 401 But many people either don't have a 401 at all, don't save enough if they do have one or spend what they save before retirement.

One of the most serious problems comes when people change jobs. When they leave, several things can be done with the money in their 401 If the company permits it, they can leave the money where it is, roll it over into another tax-sheltered retirement account, such as an individual retirement account, or spend it, after paying early-withdrawal penalties.

Unfortunately, the best long-term option, keeping the money in a tax-sheltered account where it can keep growing and adding to the retirement nest egg, is used the least.

"Of those leaving a job prior to retirement, only 13 percent roll it over" into a new tax-sheltered account, says Dallas Salisbury, executive director of the Employee Benefit Research Institute in Washington. "Thirty-four percent immediately spend all of it on consumable items." Another 30 percent use the money to pay off credit card balances or pay down mortgage debt or other loans. The rest put the money into other investments, after paying the penalty, Salisbury said.

Not surprisingly, older people do a better job of rolling over retirement funds than younger workers, but not well enough, Salisbury says. Almost 60 percent of people age 55 or over did not put the money into an IRA or similar account when they changed jobs, he said.

"The numbers really are frightening," agrees James H. Gately, senior vice president in the institutional division of the Vanguard Group of Valley Forge, Pa.

Vanguard, like most major mutual funds and brokerages, including Fidelity, Scudder, T. Rowe Price, Merrill Lynch and John Hancock, have been pursuing the 401 and 403 markets in search of long-term, stable sources of money for their funds.

With people living longer and the costs of retirement growing, Gately says, "people should keep the money in a tax-deferred account as long as they can, up to age 701/2, if possible," when withdrawals must begin.