When the Class of 1985 graduated, Harvard bid an official, though silent, adieu to the age-old General Education requirement.

Few undergraduates probably noticed as the 13-odd Gen Ed courses--once the staple of a Harvard education--became an academic frill. This is because nearly all Harvard students have known only Gen Ed's successor, the highly touted Core Curriculum, which entered the course guide seven years ago.

Gen Ed courses, which come close to exemplifying Harvard's promised "diversity," are now strictly electives taken for the most part solely out of interest in the subject matter. And although the courses won't give most students concentration credit toward their Harvard diplomas, enrollment in most cases hasn't sagged.

The popularity of interdisciplinary education, embodied in such maverick courses as "Cancer, Science and Society" and "Conventional Warfare" has not only survived, but in many cases appears to be booming.

Two Gen Ed courses promise to be among the 20 most popular classes this year. More than 200 students this fall flocked to Gen Ed 105, "The Literature of Social Reflection," and Gen Ed 176, "Business in American Life," a phenomenon many attribute to the courses' broad, interdisciplinary approach, a rare commodity at Harvard.

But other Gen Ed classes have seen their enrollment dwindle since the Core debuted in 1978. Professors in some of these courses say the death of the Gen Ed requirement is to blame, and some have said they will try to muscle their courses into the prosperous Core.

Students taking Gen Ed 159, "The Nuclear Age," and Gen Ed 180, "Environmental Quality and Its Management," this year will number only half those enrolled in the courses four years ago.

"Registration has fallen in half, and we're unhappy," said Mallinckrodt Professor of Biochemistry Paul M. Doty of Gen Ed 159.

"The popularity of the subject has increased in the last few years, and we're wondering why registration has dropped," Doty said. As a result of the popularity decline, Doty and his fellow teachers said they plan to try to persuade the Core office to adopt the course on the Nuclear Age into its curriculum.

Another professor who might similarly court the Core office is Stanley L. Cavell, Cabot Professor of Aesthetics and the Theory of Value, who teaches Gen Ed 112, "The Art of Film."

"I've felt very free in the Core to do exactly what I wanted to do. But as I'm a missionary of the film, and if this is second class status, then we should make a claim for first class status in the Core."

A similar claim for Core status has already been made by Erich Goldhagen, lecturer in Jewish Studies. His popular class, Gen Ed 136, "Explaining the Holocaust and the Phenomenon of Genocide," became Social Analysis 19 in 1983. But because it didn't exactly fit in the Core, it moved back to Gen Ed, where it has continued to draw large numbers of students. Enrollment did, however, drop slightly as a result of the switch.

"I feel absolutely no different. I'm not conscious that I'm teaching in Gen Ed," Goldhagen said.

But some professors face a clear dilemma in moving into the Core or department offerings.

James N. Butler, McKay Professor of Applied Chemistry, has seen the enrollment in his class, Gen Ed 180, "Environmental Quality and Its Management," drop from more than 200 students during Gen Ed's heyday to 24 last year. Despite the drop, Butler will probably remain in Gen Ed.