Workers in the private sector who have participated in a single-employer pension plan for at least five years may be fully vested under new provisions effective in 1989, says the U.S. Department of Labor's Pension and Welfare Benefits Administration.

Under the Tax Reform Act of 1986, which amends the Employee Retirement Income Security Act, workers may be fully vested - entitled to receive retirement benefits - after completing five years of service, effective after Dec. 31, 1988.An alternate plan is for workers become 20 percent vested in three years, then 20 percent each year and 100 percent after seven years.

Under ERISA's current rules, which will be replaced by new vesting provisions, participants are fully vested after 10 years of service (with no vesting before that).

They also may be 25 percent vested after five years, plus five percent for each additional year up to 10 years, and an additional 10 percent for each year afterward with 100 percent vesting after 15 years, administration officials said.

A third option under current ERISA rules is based on age and years of service of participants.