Ever since they made a marketing blunder of Edsel magnitude, it has not been easy for the makers of Coca-Cola.

Now, as if the company did not already have enough problems with the ill-fated new formula for its flagship soft drink, researchers at no less than Harvard Medical School find New Coke fails in yet another respect -- sperm-killing ability when used (foolishly) as a contraceptive douche.

And though this is not the sort of thing that could lead to a new version of the Pepsi Challenge, the researchers said they found marked differences in the ability of four different Coca-Cola formulas to act as a spermicide. At the same time, they warned against the use of soft drinks of any kind as douches after intercourse to prevent pregnancy. While there are differences among soft drinks, all fail as effective contraceptives, the researchers noted.

To test Coke's sperm-killing abilities, the three researchers prepared test tubes containing small samples of carefully preserved sperm and poured in small amounts of Diet Coke, New Coke, caffeine-free New Coke and Classic Coke -- repeating the test three times for each soda.

All of them killed some sperm, but New Coke turned out to be least effective, with Diet Coke having the most pronounced effect overall and Classic Coke recording a five times greater sperm-killing rate than its upstart rival. "Coca-Cola products do appear to have a spermicidal effect," the study deadpanned. "Furthermore, our data indicate that, at least in the area of spermicidal effect, 'Classic Coke' 'is it.' "

The team concluded that the variations in the Cokes' spermicidal capabilities apparently have to do with some hidden nuance of the secret formula for what is now called Old Coke. The formula has been guarded jealously for decades.

The improbable conclusion about Coke, reached by three researchers who admitted they undertook the study as much because they thought the premise funny as anything else, is being published today in, of all places, the New England Journal of Medicine.

"This is not Nobel Prize-winning work," confided Dr. Sharee Umpierre, a Harvard research fellow in obstetrics and gynecology who headed the three-person team, which included one other fellowship holder and a full professor. Umpierre said the study took less than an afternoon to complete.

The new study brought guffaws from reproductive health experts across the country who agreed that soda pop douches are not widely employed among young American women. But the same experts were divided on the extent to which such a belief has existed in medical folklore in the United States, though unanimous in their opinion that douching to prevent pregnancy is totally ineffective.

"They must have strange people on the East Coast," observed Dr. Larrian Gillespie, a Los Angeles urologist. "And they think California people are kinky. No way."

But Dr. Gerald Bernstein, a University of Southern California expert, said he remembers the Coca-Cola douche as a fixture of southern California teen-age life in the 1960s.

"It was the thing you did on the beach," Bernstein said.

"Yes it was humorous, but scientifically sound," Dr. Joseph A. Hill, another of the collaborators, said yesterday of the test. He did admit that the 1960s rock group the Fugs was partly responsible for the trio's rash afternoon of research.

First, one of the three came across a reference to contraceptive Coke in a standard tome on birth control. The three did not believe the suggestion that it might work. They checked the scientific literature and could find nothing under Coke contraceptives.

The only reference they could think of was a song by the Fugs. They did a song called "Coke Douche," Hill said. At that point, they decided to see if this bit of folklore had anything to it.

Coca-Cola saw little humor in the Harvard project. A spokesman said the company had not seen the new report, but "our position is we do not promote any of our products for any medical use."

As for the Harvard researchers, Umpierre professed consternation when contacted by a reporter.

"Oh, my God," she exclaimed, "I hope my mother doesn't read this!"