ASHINGTON, April 4 - A unanimous Supreme Court on Monday ruled that federal bankruptcy law shields individual retirement accounts from creditors. The decision gives middle-income consumers, for whom an I.R.A. is often the most significant retirement asset, the same protection in bankruptcy that higher-paid workers receive for their 401(k) plans and company pensions.

Those company plans, as well as Social Security payments, were already exempt from creditors. But the federal bankruptcy code's treatment of I.R.A.'s was ambiguous enough to have resulted in much uncertainty and conflicting lower-court decisions. In Monday's opinion, written by Justice Clarence Thomas, the Supreme Court overturned a 2003 ruling by the United States Court of Appeals for the Eighth Circuit, in St. Louis.

The decision was a victory for an Arkansas couple who sought to shield $65,000 in their two I.R.A.'s when they filed for bankruptcy in 2001. The couple, Richard and Betty Jo Rousey, had both worked for the Northrop Grumman Corporation, which required them to take lump-sum distributions from the pension plan when they left. They rolled over the proceeds into the two I.R.A.'s.

Several years later, the Rouseys filed for bankruptcy under Chapter 7 of the bankruptcy code, in which a court-appointed trustee supervises the sale of any assets that are not exempt under the statute and distributes the proceeds to creditors. The bankruptcy court, an appellate bankruptcy panel, and the Eighth Circuit all agreed with the trustee, Jill R. Jacoway, that the Rouseys' I.R.A.'s were not exempt and were thus available to creditors.

The Supreme Court's decision overturning the Eighth Circuit will have only limited importance if the current bankruptcy bill becomes law. The bill, which passed the Senate last month and will be taken up later this week in the House of Representatives, generally makes federal bankruptcy law less protective of consumers. But it does contain a provision that generally addresses "protection of retirement savings in bankruptcy," which includes I.R.A.'s among the retirement accounts to be shielded from creditors.

Brady C. Williamson, a bankruptcy law specialist in Madison, Wis., said in an interview that this protection would become all the more important given the strictures placed on consumers by the overall bankruptcy bill being considered by the House. He opposes the bill.

Protecting I.R.A.'s, he said, "validates the core premise of bankruptcy law, which is that you get a fresh start, not from scratch, but with the ability to keep some core assets out of the hands of creditors."

The case, Rousey v. Jacoway, No. 03-1407, was argued more than four months ago, and neither the argument nor Justice Thomas's opinion referred to the pending legislation.

Instead, the opinion parsed the language of the current bankruptcy code, which exempts "a payment under a stock bonus, pension, profit-sharing, annuity, or similar plan or contract on account of illness, disability, death, age, or length of service."

That provision required the court to answer two questions. First, are payments from an individual retirement account made "on account of age?" And second, is an I.R.A., which is not mentioned in the statute, a plan similar to those that are specifically named (stock bonus, pension, profit-sharing, annuity)?

The court answered yes to both those questions. Justice Thomas said that the 10 percent tax penalty that applies to withdrawals from an I.R.A. before the age of 59½ meant that Congress intended to deter early withdrawals and encourage people to use these accounts as intended- for retirement.

The 10 percent is a substantial penalty, he said, noting that it did deter all but a small fraction of account holders. Consequently, Justice Thomas concluded, an account holder's unrestricted right to the assets in an I.R.A. depends on age, and the right to payment was "on account" of age.

Answering the second question, the court said that an I.R.A. was similar to the pension plans that the statute explicitly exempts because, like those, an I.R.A. has "the same 'primary purpose,' namely Americans to save for their retirement" by providing a substitute for the wages earned while working. Justice Thomas noted that there was a severe tax penalty for those who did not begin to withdraw money from an I.R.A. as required during the year after turning 70½. This provision also emphasized the use of I.R.A.'s as retirement accounts, he said.

In a second case related to bankruptcy on Monday, the court agreed to decide whether Congress had the constitutional authority to make states liable to suit under the bankruptcy law. This issue, raised in an appeal brought by four public colleges in Virginia, has been on the court's docket before, but the justices failed for procedural reasons to resolve it.

States often find themselves in bankruptcy court as creditors or debtors of a bankrupt company, in this case, a college bookstore. Wallace's Bookstores operated a chain of college stores and, after filing for bankruptcy in 2001, filed suit in Kentucky to collect money it said that it was owed by the colleges.

The colleges - Central Virginia Community College, Virginia Military Institute, New River Community College and Blue Ridge Community College - jointly raised the state's defense of sovereign immunity from suit. But a federal bankruptcy court in Kentucky, in a decision affirmed by the United States Court of Appeals for the Sixth Circuit, in Cincinnati, rejected the immunity claim and refused to dismiss the suit. The case, Central Virginia Community College v. Katz, No. 04-885, will be argued next fall.