The 1978 espionage case of U.S. Information Agency employe Ronald L. Humphrey and antiwar activist David Truong was part spy story and part love story.

Prosecutors at the trial in U.S. District Court in Alexandria charged that Humphrey fed classified USIA documents to Truong -- who had ties to Communist Vietnamese officials -- in exchange for the hope of Truong's help in freeing Humphrey's bride-to-be from Vietnam.

"I don't believe we ever tapped Humphrey's phone because Truong was the spy and Humphrey was the feeder," said former U.S. Attorney William B. Cummings in a 1982 interview when the two men started serving 15-year prison terms.

Today, Truong's and Humphrey's lives still are intertwined, but lately it is Humphrey who has been painted as the heavy in the case.

Last September, the U.S. Parole Commission told Truong that he can expect to be freed from prison in August 1986, after serving about five years of his sentence. The commission informed Humphrey in February, however, that he will be required to serve 10 years.

"It's a terrible, just a brutal injustice," said the Rev. Dr. George Hill, pastor of Calvary Baptist Church in the District of Columbia, who has befriended Humphrey, his Vietnamese wife, Kim, and the couple's three children.

A staff lawyer for the Parole Commission explains the difference in treatment by noting that Humphrey is an American and that he held an official government position with a high security clearance. "Truong could never have done anything with this information without Mr. Humphrey," the attorney said.

Some other former government officials view the case differently, but so far to no avail. Hill said that letters from former president Carter, former attorney general Griffin Bell and former prosecutor Cummings, all urging that Humphrey be paroled after five years, fell on deaf ears at the Parole Commission.

Meanwhile, the former codefendants see each other often. Both are incarcerated at the medium-security federal prison in Danbury, Conn., where each works at a clerical job. Humphrey, 49, declined to be interviewed. Truong, 39, said in a telephone interview that he looks forward to leaving the United States, probably for Paris, upon his release.

Truong said he often sees Humphrey jogging, and that the two men talk amicably. Asked if they discuss their respective parole situations, Truong said, "Not very much."