The Supreme Court agreed yesterday to decide whether a 122-year-old law, designed to help veterans by putting a $10 ceiling on the amount they could pay lawyers in disability claims cases, has the unintended effect of preventing veterans from hiring lawyers to pursue those claims.

The law was passed during the Civil War to prevent unscrupulous lawyers from taking advantage of veterans who needed help in filing simple pension claims. But a federal judge earlier this year ruled that the law unconstitutionally prevented veterans from challenging complex Veterans Administration rulings and procedures.

Under the law, lawyers face criminal penalties if they accept more than $10 from a veteran to help him pursue a claim. As a result, some veterans say, they frequently are forced to represent themselves or accept limited help from volunteer organizations that have limited resources.

The Justice Department appealed the ruling to the high court. The department argued that Congress had determined that the VA claims process was "informal and nonadversarial," that VA officials help veterans with their claims and that voluntary organizations provide adequate representation.

Justice William H. Rehnquist last September blocked the lower court decision, so the law remains in effect pending a decision by the Supreme Court this spring.

In their final actions before taking off for a four-week holiday recess, the justices issued a series of opinions in five minor criminal cases, ruling for the prosecution in four of the five. The court:

* Ruled, 9 to 0, that a federal trial judge acted properly when he allowed prosecutors to discredit a witnesses' testimony by showing that the witness belonged to a secret prison gang whose members have sworn to commit perjury for each other. Rehnquist wrote the opinion in U.S. v. Abel.

* Ruled, 6 to 3, that Congress, in passing a law making it a crime to rob mailmen, also intended to make it a crime to rob other federal agents such as Secret Service agents. Rehnquist, writing for the majority in Jose Garcia and Francisco Garcia v. U.S., said congressional intent was clear. Justice John Paul Stevens, joined by Justices William J. Brennan Jr. and Thurgood Marshall, dissented.

* Ruled unanimously that a defendant, convicted by a jury on some counts of conspiring to sell drugs, but acquitted on other counts, has no right to appeal on the basis of inconsistent verdicts. Rehnquist wrote the opinion in U.S. v. Powell.

* Ruled, 8 to 0, that a defendant must testify at his trial if he wants to later challenge a judge's ruling allowing the jury to hear evidence of the defendant's prior convictions. Chief Justice Warren E. Burger wrote the opinion in Luce v. U.S. Stevens did not participate.

* Ruled that police in Illinois acted improperly when they did not stop questioning a suspected armed robber after the man indicated that he wanted to have an attorney present. Rehnquist, Burger and Justice Lewis F. Powell Jr. dissented from the unsigned majority opinion in Steven Smith v. Illinois.