OSTENSIBLY, union leaders have one purpose - to protect and advance the interests of their members, push ing for higher wages, better health and pension benefits, etc. But the nation's top teachers union is doing just the opposite - exploiting its members by recommending sub-par retirement plans, for the union's own profit.

New Yorkers may recall how then-Attorney General Eliot Spitzer last year sued the benefits arm of the state teachers union for accepting nearly $3 million a year from an investment firm in exchange for exclusively endorsing a high-cost retirement plan. Teachers, according to Spitzer's suit, "believed that the union was vouching for the quality" of the plan.

The state union eventually settled for $100,000 and agreed to ensure that its members were made aware of any compensated endorsements - a news item that could not have escaped the attention of the National Education Association (NEA), which represents more than 3 million teachers (including those in New York). Yet the NEA continues to be involved in the same sort of scheme.

Rather than steering members toward the best retirement plans, the NEA's leadership is quietly accepting payments to endorse a low-return, high-fee plan that eats away at the savings of the nation's public schoolteachers.

Not including management fees, the NEA's only officially endorsed "retirement program" - the Security Benefit Life Insurance Corporation's Valuebuilder annuity - charges 0.9 percent to 2.6 percent a year. Throw in management fees, and the least expensive option costs a teacher 1.73 percent of her account balances each year, while the most expensive costs 4.85 percent.

Over time, a fee that large is devastating. Without inflation, the educator would have to earn nearly 5 percent each year simply not to lose money. Consider a teacher who socks away $500 a month and earns an average yearly return of 10 percent for 35 years: She'd wind up with $1,788,760 upon retirement - quite a sizeable nest egg. But if she were paying 4.85 percent in fees, she'd accumulate less than one-third as much - just $587,854.

It appears that the NEA is willing to endorse a shoddy plan in exchange for a contribution to its coffers. In 2004, the union collected nearly $50 million from the investment vehicles it endorsed.

The investment firms, of course, more than recoup their money by selling their products. Many statewide teacher unions that refer members to the endorsed plans get a share of this revenue, as well.

Thankfully, some have taken notice of this pay-for-play system. This past summer in Washington, two teachers filed a federal lawsuit on behalf of the 57,000 schoolteachers who've invested $1 billion in the Valuebuilder plan.

The suit argues that the schoolteachers who invested in Valuebuilder did so only because of assurances that the NEA had "conducted an extensive review of numerous financial services companies to find the best provider." But the NEA's real motive in endorsing the plan, according to the suit, was the money it was paid to do so.

For compensation, the plaintiffs are seeking the fees the NEA received for its endorsement. It's a shame these teachers have to sue to get their money back.

If the NEA's true goal were to help its teachers, it would voluntarily return any money it received for recommending bad investments. But I wouldn't bet the sandbox on it.