Thanks to the big May jump in the stock market, government workers in the stock market option of their thrift savings plan earned 9.44 percent on their money last month. The bond market option of the tax-deferred savings plan earned 2.80 percent while the steadier Treasury securities option earned 0.76 percent.

More than 1.5 million employees, over half the federal work force, invest in the savings plan. It is worth

$6 billion and is growing by

$10 million each workday.

In April, both the stock and bond market options of the savings plan were losers, down 2.52 percent and 0.94 percent, respectively. The Treasury option paid 0.71 percent for that month while the high-risk,

high-reward options were digging themselves into a hole.

For the 12-month period ending in May the C-Fund (stock market option) had a rate of return of 16.26 percent. The F-Fund (bond market option) earned 8.49 percent, while the G-Fund (Treasury securities) earned 8.59 percent.

Most of the money invested in the savings plan is in the G-Fund, although Congress is set to broaden employee investment options.

Tracking the C and F funds of the federal investment program shows the fluctuations of Wall Street and the dangers of making investments based on short-term performance.

In January the C-Fund lost 6.59 percent. In February

it gained 1.26 percent.

In March it gained 2.64 percent. In April it lost 2.52 percent, and last month it gained 9.44 percent.

Meanwhile, the F-Fund lost 1.38 percent in January, gained 0.21 percent in February, gained 0.01 percent in March, lost 0.94 percent in April and gained 2.80 percent in May.

The steady G-Fund earned 0.68 percent in January,

0.64 percent in February,

0.72 percent in March,

0.71 percent in April and 0.76 percent in May.

At present, only workers under the new Federal Employees Retirement System can invest in all three options. The matching contribution they get from the government, however, must be invested in the Treasury option. The bulk of the work force, which is still under the old Civil Service Retirement System, can invest only in the Treasury option.

The Senate is expected to approve, perhaps this week, legislation that would let all federal workers invest in any or all of the three options. The bill has cleared the House and has White House approval. If it becomes law, it would mean workers could invest all their contributions, plus any government contributions, into any of the three options.

Federal workers have until July 31 to sign up for the savings plan or make changes in their accounts.

Workers under the new retirement system pension plan can invest up to 10 percent of their pay and get a 5 percent match from the government. Those under the old civil service retirement plan can invest 5 percent of salary. They do not get a matching contribution from the government.