Supreme Court Justice Thurgood Marshall yesterday warned American blacks in strong terms against accepting "the myth" that things are improving economically and socially for them.

"Be careful of the people who say, 'You've got it made.Take it easy. You don't need any more help'," said Marshall, the only black on the Supreme Court, in a speech at Howard University that broke a self-imposed absence of more than a decade from making public statements on racial issues that might come before the high court.

While other black leaders - including members of the Black Congressional Caucus - have made the point recently that black gains are more illusory than real, Marshall's speech carries added weight because its words were so much more blunt and forceful than Supreme Court justices generally use when discussing public issues.

The speech was an emotional occasion for Marshall, who returned to the place where he honed the legal argument she used for more than 21 years as a lawyer fighting segregation.

The audience contained many companies from those fights and his speech, while biting, had some of the informality of a college bull session.

Yesterday's speech came 25 years after Marshall, as counsel for the NAACP Legal Defense and Education Fund, headed the team of lawyers who successfully argued before the Supreme Court the landmark case that outlawed segregation in the nation's public schools!

"Today we have reached the point where people say, 'You've come a long way'," said Marshall. "But so have other people come a long way.

"Has the gap been getting smaller? No. it's getting bigger. People say we're better off today. Better off than what?"

Marshall said he is urged to travel the country to give black children "inspiration."

"For what? These Negro kids are not fools. They know that if someone says they have a chance to be the only Negro in the Supreme Court, the odds are against them."

When he does travel, he said, people tell him the same troubles they did 20 and 30 years ago when he first sounded the note that "things are going to get better."

"Only guess what I'm getting now?" he asked. "You not only told me that but you told my father too and he's no better off. Are you going to tell my children that too?"

He warned that "they" - antiblacks - "in every phase of American life are still laying traps for us." He pointed to reports of a resurgence of the Ku Klux Klan and said, "The Klan never dies. They just stop wearing sheets because sheets cost two much."

He urged American blacks to train themselves to be better than the whites they go up aginst in law, politics or business. Quoting Charles E. Houston, the legendary dean of the Howard Law School during the 1930s, Marshall said, "You've got to be better, boy. You've got to move better."

He said that American blacks now need to use both the courts and their political muscle to end inequality.

"Don't listen to that myth that it [inequality] can be solved by either or that it has already been solved. Take it from me, it has not been solved."