Although the new civil service tax-deferred investment plan is considered eye-poppingly generous by private industry standards, cautious U.S. workers have been relatively slow to join it. The federal version of the thrift plan allows workers to contribute up to 10 percent of pay and get another 5 percent contribution from the government.

Few private sector plans allow such a high percentage-of-pay contribution, and few private employers will match the amount Uncle Sam kicks in to workers' accounts.

Since the program started in April, only 425,510 of 2.7 million feds have signed up for payroll deductions to the investment plan, as of June 6. Nearly half of all accounts contain only government contributions --

equal to 1 percent of salary for feds hired between 1984 and last year -- with no additional contributions from employes to their accounts.

Officials managing the investments hope that participation jumps during the current open season, and during another period later this year, for people making 1988 investments.

Although the program is growing by $1 million a day and has the backing of all major federal and postal employe organizations, the relatively low rate of participation has surprised many experts who analyze such investment programs.

If participation rates stay low, the percentage of pay upper-income ($50,000 per year or more) workers can contribute in the future will be affected. Under rules governing such plans, the amount upper-income workers contribute is linked to the average contribution rate of all eligible lower-income workers. All such plans limit the maximum annual employe contribution to $7,000.

Tax-deferred thrift plans for some state-local government workers and to nonprofit organizations usually allow employes to put in a greater percentage of pay.

All money is handled by the new Federal Retirement Thrift Investment Board. This year everything goes into the so-called G-Fund made up of long-term Treasury securities. This month the fund is paying 8 5/8 percent, in May it was 8 3/8 percent, and in April the rate was 7 5/8 percent. If the board thinks that the monthly interest rate will drop, it can lock in current contributions to the higher rate of return.

Because thrift plan contributions reduce gross income, workers pay lower taxes with each paycheck.

Employes leaving before they are eligible to retire can take their total account funds and put them into an Individual Retirement Account or a qualified pension plan.

People covered by the Federal Employees Retirement System (nearly everyone hired since the end of 1983) may put in 10 percent of pay (or a maximum of $7,000 per year) and get a 5 percent tax-deferred matching contribution from the government. Most pre-1984 hires covered by the old Civil Service Retirement System can put in 5 percent but get no match from the government unless they switch to the new FERS plan during an open season that starts next month. As of June 6, data show that:

Of the 831,107 accounts, 556,876 are for workers hired since 1983. But only 151,279 of them (27 percent) have put in any money of their own.

Of the nearly 2 million feds in the old pension plan, 274,231 have opened accounts.

As of June 10, the thrift plan was worth $292 million, including $81.3 million from employe contributions, $34.7 million from the government and $2.9 million in earnings. The remainder of the fund consists of retroactive contributions the government made to start employe accounts.