LONDON - Among the great mysteries of England there is Stonehenge, the large assortment of cut stones that for centuries has confounded archaeologists, astrologers, astronomers, historians and theologians - all of whom have researched the grounds and after thoughtful contemplation developed detailed theories of how the configuration came to be and just what spiritual, scientific or humanistic significance it might possess.

Then there is the true great mystery of England: Marmite spread.

And this one really has people confused.

Marmite's origins are known, its roots well documented. But unexplainable by conventional thought is how Marmite has survived. This year marks the 100th anniversary of the bread spread - it's a yeast extract, if pointing that out doesn't affect your breakfast too much - and the collective question in England seems to be: Is this something worth celebrating?

It is, of course, to Marmite's makers, who revel in the fact that a large percentage of the English loathe the food, which resembles spent motor oil in texture, appearance and, some would argue, taste.

But the makers quickly point out that Marmite can be found in more than one in four kitchens in England, and that for every Marmite hater there's a Marmite lover who could not imagine eating his or her morning toast without first slathering on the viscous brown spread.

Marmite has become as identifiably British as red double-decker buses, the silhouette of its squat jar as recognizable here as that of Big Ben.

With Marmite's 100th birthday under way, its producers, Bestfoods Ltd., are unashamedly pushing it at every turn - with commemorative jars, testimonials from members of the royal family, motions in the House of Commons, and an advertising campaign acknowledging that consumers of the product would be kind to double up on the morning mouthwash.

"There is really no fighting the fact that some people absolutely hate the stuff," says Sabrina Lynch, a Marmite spokeswoman who confesses to eating it. "This is a tongue-in-cheek way of acknowledging that people either love it or hate it."

Marmite has been produced since 1902 in Staffordshire, just a few staggers away from the Bass Ale brewery in Burton-on-Trent, which is no coincidence. The basic raw material used in the making of Marmite is spent brewer's yeast, a substance whose original and only use was to ferment sugars into alcohol.

After Marmite got off the ground and into the jar, it was discovered that the new food was also rich in a number of B vitamins. It became a staple of meals for British soldiers in World War II - although Bestfoods acknowledges that soldiers were as likely to spread Marmite under the zipper area of their pants to stave off infection as they were to eat it.

The acknowledgment is typical of Bestfoods, which has found no point in denying that some people would rather put their lips to Prince Charles than to Marmite. With the beginning of the centennial year came a television campaign letting consumers know that, well, if they wanted to hate it, that was fine. Just as long as its lovers went unharassed.

One commercial opens at a freak show in Blackpool, set in the year Marmite was born. A barker standing in front of a curtain hypes the next act: "Ladies and gentleman, life is full of surprises," he bellows. "Confront your primitive fears! The time has come to gaze behind the curtain! Behold - the sheer inhumanity!"

At that, the curtain is pulled back to reveal "Marmite Man," a rather hungry bloke spooning Marmite from a jar into his mouth. Crowd shots are shown. Some are aghast. Some seem to want a spoonful. Some are so upset that they burst into tears.

Oliver Bradley, the company official responsible for pushing Marmite, says the advertising campaign only made sense. Marmite is as common in England as the Sunday roast, he says, so there is no sense trying to persuade people that the product is something it's not.

"We know that for 100 years, a whole lot of people have hated it," he says. "Our brand, unlike others, has the confidence and cheekiness to say we 'we know that, and we don't care.'"

Apparently recognizing that eating Marmite constitutes a challenge of sorts, a motion praising Marmite in the House of Commons was left for the minister for sport to introduce. "We take intense satisfaction from the essential Britishness of the product and its lack of appeal for the majority of the world's population," says the motion, which was introduced by Sport Minister Tony Banks.

Bestfoods claims that the Rolling Stones, who apparently can't get no satisfaction from something like jelly, eat Marmite while touring to keep up their strength and stave off homesickness - and just look at Keith Richards.

(Bestfoods is part of Unilever, which owns the Ben & Jerry's ice cream empire. A spokesman says there are no plans for "Chunky Monkey Marmite" ice cream or any other type that would include the old yeast.)

Seamus Waldron, an Englishman transplanted to Boston, has created a Web site devoted to Marmite. The site, www.ilove marmite.com, provides history and testimonials about the spread, and Waldron says the site has had about 50,000 visitors since he created it a little more than a year ago.

He got the idea after his bosses at a computer business in Boston began breaking out bagels and jam for the employees every Friday. Raised on Marmite and a bit homesick for it, he began taking it to work.

"Really, I just got sick of Americans asking me about it, so I made the Web site," says Waldron, 32. "I've been eating it since before I can remember."

He has yet to find the American who can stomach it, and he has his theories why.

"It's the theory of nature vs. nurture," he says. "It's either a genetic thing or you're brought up with it. Personally, I think you just have to start eating it as a kid. Otherwise, you just gag."