"Someone else said this of Crossroads but it's really true," said Melanie Cates, wearing a bright pink batik caftan. "These last eight weeks have made sense of the last 21 years of my life." And, she added, it will affect "the next 21 years of my life."

Cates, a recent graduate of George Washington University who traveled to the tiny nation of Guinea-Bissau last summer with Operation Crossroads Africa, was among the 160 guests Saturday at the Washington Hilton, for a luncheon celebrating Crossroads' 25th anniversary that included a slide show, a performance on the kora and an address by actor Ossie Davis.

A forerunner to the Peace Corps, Operation Crossroads Africa was conceived by the late Rev. James H. Robinson in 1957, in the belief that sending college students to work in another culture would enhance international understanding. His wife, Gertrude Robinson, was present Saturday and told the audience, "He had an idea whose time was ripe, but it was his faith in young people" that made it work. Twenty-five years later, according to organization literature, about 5,000 Americans have gone to 35 countries in Africa and 13 in the Caribbean.

Most of the men and women present Saturday were Crossroads alumni. Like Cates, they performed such tasks as building wells and schools and teaching English, and they were filled with memories and idealism.

"We rolled in the mud together and lived with the mosquitoes," said Linda White, now the Crossroads representative here. She went during the summer of '71.

Helen Mills is a children's librarian in Bethesda, but 19 years ago she went to Mamou in Guinea as a Spanish major from Fisk University. The experience, she said, gave her many insights. "We usually had bread, jam and coffee for breakfast," she remembered. "One day the group felt like a treat and asked the cook to get them some eggs. He returned with about 30, and we enjoyed them so much we wanted more the next day. But the merchant wouldn't sell any more. He said we'd taken the town's supply and would have to wait. That was a shocker."

Many Crossroaders, as they call themselves, are black Americans who made the trip to Africa as pilgrims tracing their heritage. The Rev. Harold Lewis, rector of St. Monica's Church, went to Zaire in 1967 "to explore my roots." The beauty of it, he said, was going not as a tourist but "working side by side."

"It's a dream, Africa's a dream," said Glo Ivory, who went in 1976. "Africa is something you must touch, taste and smell before you die."

About 15 Africans visiting the United States on a Crossroads exchange attended the luncheon and, later, a reception at the residence of Kenyan Ambassador John P. Mbogua, who said Crossroads "provided the first bridge between Americans and Africans at the level of ordinary people."

Among the guests was Zhohair Mostafa, who teaches public administration at the Sadat Academy for Management Sciences in Cairo and is looking at local government here. "If we can develop the small villages," he said, "then we can develop Egypt. Cairo is overurbanized."

Lewis, attributing part of Operation Crossroads' success to the fact that, unlike the Peace Corps, it's a summer program and not a commitment of two years, said, "This predates the Peace Corps and it may survive it."