The highest rate of interest you may be able to earn on your retirement nest-egg dollars for the balance of this decade could come from a source you've probably never thought of: adjustable-rate and fixed-rate home mortgages originated in cities and towns across the country.

That's the idea behind a major new investment product to be introduced nationwide this fall. It's called the Blossom Fund, and represents the first effort by a financial services conglomerate to put thousands of Americans' retirement cash directly into home mortgages whose rates rise -- or fall -- with the market.

The Blossom Fund was dreamed up by the Lomas Financial Security Insurance Co., a subsidiary of Dallas-based Lomas & Nettleton Financial Corp. The $18 billion of home loans Lomas & Nettleton services makes it the largest mortgage banking firm in the United States.

The Blossom Fund will offer individuals high-yielding, tax-deferred, variable-annuity contracts invested in adjustable-rate, fixed-rate, graduated-payment, shared-appreciation and other forms of home mortgages. The fund will be the first commercially available investment medium allowing individual consumers to tap into the high yields of home mortgages, and to move up with those yields when rates (or inflation) accelerate.

Blossom will purchase mortgage securities with investors' retirement dollars. The securities, in turn, will represent interests in huge pools of home loans, priced to yield the same high "wholesale" rates obtained by major institutional investors.

The rates of return will vary with the marketplace year to year. Most new mortgage securities currently yield in excess of 13 percent compared with ordinary money market rates of between 10 and 11 percent.

Small-scale retirement investors normally are locked out of the market for higher-yielding mortgage securities because the minimum price of entry is so high. So-called Ginnie Maes, which are mortgage certificates backed by pools of government-insured home loans, have a minimum price of $25,000. The Blossom Fund will cut individuals into the mortgage securities marketplace for a $2,500 minimum investment, with a $500 minimum for any subsequent additions to a retiree's account.

Individual Retirement Accounts, Keogh plans and corporate retirement plans may invest in the fund, pending state regulatory approval. Twenty-two states already have approved Blossom variable-mortgage annuities for IRA accounts, according to a Lomas executive. Securities regulators in 30 states have given the green light for the fund to be offered to the general public.

Because retirement accounts accumulate capital on a tax-deferred basis, the high mortgage yields generated by Blossom could be the equivalent of 24 to 28 percent annual taxable yields. A 14 percent, tax-deferred return from a mortgage pool, for example, would be the same as 28 percent on a taxable investment for a taxpayer in the 50 percent marginal bracket.

The investor eventually will be taxed on his or her returns, but at the lower rates most taxpayers face following retirement. Investors who pull dollars out of the fund prior to the federally mandated minimum age of 59 1/2 may be subject to a 5 percent federal tax penalty, as in any retirement fund. Annual administrative costs to the investor will range between $40 and $50, plus a 1.4 percent "advisory" fee to Lomas for running the fund.

What makes the Blossom Fund's introduction this fall so significant?

First, it is symbolic of where the U.S. home mortgage market will be heading. Rather than depending upon depository institutions such as banks and savings and loans for capital, the marketplace will be turning increasingly to individual consumers, whose mega-billions of dollars of retirement funds represent the nation's fastest-growing financial resource for housing.

Second, the Blossom concept offers the retirement investor a choice of fixed-rate or adjustable-rate loans.

The fund allows investors who believe that rates in the capital market are likely to head up in coming years to plunk their dollars down on adjustable-rate loans. Because the majority of such mortgages originated by Lomas & Nettleton carry no life-of-the-loan rate caps, or ceilings, according to Geri Graham, Blossom's national product manager, the yields on these mortgages could escalate significantly in the coming decade.

On the other hand, investors who think mortgage rates have seen their cyclical peak can lock into fixed-rate mortgage securities in 1984, and sit pretty for years.