Donald Fisher was only 9 when Martin Luther King Jr. was shot to death on the balcony of the Lorraine Motel in Memphis 15 years ago today.

But to remember the slain civil rights leader who inspired him to higher learning as a path to "promoting social change," Fisher, 24, an electrical engineer, drove south from North Carolina to visit relatives and watch King's widow lay a wreath on his grave.

"You can't make changes unless you can fit into the mainstream," said Fisher. He earns $20,000 a year as an engineer for Duke Power--more than his father ever dreamed possible as a tobacco farmer with five children and two acres.

"It makes me sad that such a great black leader had to die violently," he said, as Coretta Scott King set yellow chrysanthemums by the grave. "It makes you realize that you can't make changes unless you understand how the system works."

Then he bowed his head along with 75 others who gathered here at the gleaming marble crypt--and thousands at similar ceremonies across the nation--to pray and remember on the 15th anniversary of King's assassination.

The Rev. Joseph Lowery, head of the Southern Christian Leadership Conference, founded by King here on Auburn Avenue two decades ago, said the battle had moved from the "lunch counter to the cash register." But, tallying black progress, he found it wanting.

"We've gotten more into the mainstream only to find the mainstream polluted," he said. "Even if you eliminated race as an issue, we'd still have massive unemployment--and it would be integrated. You'd have to say the dream is in a holding pattern."

Lowery urged those who would remember King to "get out of the suites and go back to the streets" for a scheduled August march on Washington for jobs and peace.

"We have achieved radical social change," he said, "but the issues Dr. King set forth are still issues today."

Cars with loudspeakers blaring King's "I have a dream" speech cruised up and down Auburn Avenue, past his former church as well as beauty parlors and a soup kitchen.

Debate over his death still raged. About 340 people telephoned a radio station today that asked listeners, "Do you believe James Earl Ray acted alone in slaying King?" Sixty-one percent said no.

Ray is serving a life sentence for King's assassination in Brushy Mountain State Prison in Tennessee.

At a downtown rally, civil rights and labor leaders used the day to hammer at President Reagan, take stock and reminisce. A crowd of about 150 waved signs for "Jobs and Peace" and "Bread, not Bullets."

"We ought to say, 'We love you Ronnie,' but at your age, you ought to be back at your ranch," said Lowery to cheers.

"Ronald Reagan doesn't care what color we are," said a white AFL-CIO official. "The only color he cares about is green."

In the shadows were other civil rights veterans. Some had gone on to economic boycotts, others into politics. There was John Lewis, a city councilman who survived billy clubs and police dogs on the Selma-to-Montgomery march with King.

There was the Rev. James Orange, 40, a beefy presence at 300 pounds, who counts 88 arrests on various charges related to marching in the South since the 1960s. He was "rassling" with Atlanta Mayor Andrew Young just feet away from King when shots rang out April 4, 1968.

"I see people here today who were in their teens then," said Orange. "Of course, we're all gray-headed now, but it's good to see them still in the struggle."