A collective sigh of relief may be heard July 23 emanating from investment advisers, venture capitalists, small business entrepreneurs, insurance brokers, and maybe even from such unexpected sources as options traders and purveyors of objects d'art.

On that day the Department of Labor's prudence regulation covering the investing of pension funds will go into effect. Henceforth, a manager placing monies held in trust for plan participants and beneficiaries cannot be found personally liable if a particular investment turns sour, provided the manager has abided by three department guidelines.

These require the investor to consider the diversification of risk, liquidity and current return relative to anticipated cash flow, and projected return relating to funding objectives. If these criteria are followed, the investment manager can assume a "safe harbor;" i.e., that he or she is in compliance with the law and is not likely to be sued.

Since 1975, Labor has brought only one imprudence suit against an investment adviser. Last spring it sued the Unicorn Group, a New York investment-management firm for investing $4 million of Teamsters pension fund money in 1975 in companies connected with swindler Barry S. Marlin. He pleaded guilty last year to federal fraud charges after raising about $50 million from thousands of investors in a variety of alleged Ponzi (pyramid) schemes. The Labor Department still hopes to recover some of the Teamsters money.

The Unicorn case has been stayed, pending a federal grand jury investigation. Last April Walter Peters, a principal partner of Unicorn, attacked the Labor Department for bring suit based on a single investment and ignoring the general portfolio performance, which he claimed was the "highest in the history" of that pension fund.

Contrary to Peters' argument, a Labor attorney said this week, Unicorn's investment would clearly be covered by the new regulation.

In the total portfolio approach to investing adopted by the government, the adviser is judged on the entire barrel of apples, not a single rotten fruit. However, just because a fund manager is not precluded from making an investment solely on the grounds it is risky, that does not allow him or her to invest in "a pig in a poke," according to Ian Lanoff, administrator of Labor's pension and welfare benefit programs.

Prior to the regulation, a pension fund manager was not actually prohibited from investing in high risk ventures; he simply was expected to act like "a prudent man," a vague term open to many interpretations. Uncertainty bred stalemate.

Since passage of the Employe Retirement Income Security Act (ERISA) in 1974, fiduciaries have been so nervous about being held personally liable for imprudent investments that in a few cases pension funds have had difficulty recruiting them. ERISA's far greater consequence was the emergence of a strong conservative investment philosophy. For example, the International Foundation of Employe Benefit Plans found that 64 percent of the trustees it surveyed in 1975 were unwilling to invest in anything but blue-chip securities.

Others testifying before congressional committees traced the conservatism to the licking many investors took during the high flying stock market of the late 60s and early 70s. While there is undoubtedly truth in both arguments, the result was nonetheless economic concentration. A post-ERISA study by Georgetown University Law School professor Roy Schotland found that seven banks, six of them in New York, managed nearly one fifth of all the pension fund assets in the country.

Risk capital needed to finance nascent businesses - the Xeroxes of tomorrow, as they are often called - dried up. It is believed the new prudence standard will act as a pump primer. According to Stanley E. Pratt, publisher of Venture Capital, the amount of money available remained virtually static at about $3 billion for a decade. "Then last year the dam broke," he said. While the principal impetus was lowering of the capital gains tax, Pratt said, the 15 to 20 percent returns realized by some venture capitalists also attracted investors.

A billion new dollars in commitments has been received in the past 18 months by organized venture capital groups; pension funds, which began to jump in during the final quarter of last year, placed $50 million. Pratt said the groups are now busy raising $300 million, a far greater share of which is expected to come from pension funds. "If we could get just one percent of their ( $550 billion plus) assets, we could double our capital base," Pratt sighed.

Although the Labor regulations do not specify what types of investments are approved as prudent, the preamble lists certain examples: small businesses, index funds, guaranteed fixed interest rate insurance contracts, art such as paintings and antiques, precious metals and other collectibles. At a press conference to announce the regulations, Lanoff also added options and foreign investments. This smorgasbord of investments should be sufficient to bring relief from a straight blue chip diet.

In related news, the Labor Department recently issued a study quantifying the retirement benefits denied workers between 1942 and 1974, the year ERISA was enacted. The legislation came about largely due to horror stories told by pension plan participants or the whims of their employers, or who were fired just prior to retirement age. Yet ERISA was not made retroactive, so many were thought to be permanently out of luck.

As the result of pressure from the Gray Panthers, an activist group of older people, and other organizations, Hay Associates did a study for Labor in which they calculated that 83,500 persons now surviving lost $207.8 million in defined benefits. (Defined contribution plans like profit-sharing are not covered by the study.) Were benefits to be paid (without interest) to these persons until their deaths, the average amount per year per capita would be $446.

Total annual payout by the government would amount to approximately $20.6 million in 1979, increasing to $22.6 million a year by 1984. Thereafter payments would decrease yearly until all the beneficiaries died. Less than 30,000 are expected to survive after the year 2000.

Before the study, even proponents of retroactivity had supposed the sum would be astronomically high. But the estimates have revived the possibility of legislation to restore some of these lost benefits.