"Rely," declares the label on the box of tampons. "It even absorbs the worry."

That advertising promise has a tragic irony for the women struck by the new, baffling and sometimes deadly illness called toxic shock syndrome.

And it also is tragically ironic for Procter & Gamble Co., which spent a decade developing the product and an estimated $10 million launching it into the intensely competitive world of "catamenial" products, as the industry calls tampons and sanitary napkins.

Just how Rely and toxic shock syndrome are connected is not yet known. But it does appear that Rely -- like toxic shock syndrome -- is uniquely a product of our times.

Rely, first introduced in 1974, was the first of the "superabsorbent" sanitary products. It meant a breakthrough in a product line that hadn't changed much since 1938, when Tampax first introduced a tampon, a product that a woman could wear internally during her menstrual period.

To hear the industry tell it, Rely was a response to an aspect of the woman's revolution of the 1960s and 1970s.

"My feeling was, it's a changing world," said Mary Helen McGuire, an executive with Johnson & Johnson Co., a leading manufacturer of sanitary napkins.

"Ladies all of a sudden began to get out more and they were doing more things. They were more self-sensitive, more aware of themselves," she said during a court appearance in a lawsuit last year between Procter & Gamble and Johnson & Johnson.

Rely reflected that new world, offering women more security and thus, more freedom, she said.

Tampax, which had had the field of tampons to itself for more than a generation, was confronted in the 1970s by one new tampon product after another, all promising extra absorbancy.

But Rely was unique. The traditional tampon was made of cotton and rayon strands, but Rely contained two components, a common polyester foam chopped into small pieces, and chips of an organic absorber called carbonmethylcellulose -- a chemical polymer whose chain-like structure contains space to hold molecules of liquid.

There is no reason to suspect that carbonmethylcellulose is a potentially toxic product -- it is the active ingredient in popular super-absorbent paper towels, for example, said Dr. Bruce Dan, an investigator with the Center for Disease Control in Atlanta who is searching for the answers to toxic shock syndrome. The substance had never been used as a tampon, however.

Rely's inventors also came up with a totally different shape. McGuire describes it as a tea bag. Unlike the traditional tampon, which expands lengthwise, Rely expands radially, P&G says, providing a better "fit" and thus giving women more protection. Hence the name, Rely.

Rely was a success. After test marketing in 1974, the sales area was gradually expanded throughout the country. By this year, Rely was a national product. More than 240 million units were sold last year, bringing P&G more $2 million in sales income.

The success of superabsorbers set off a competitive scramble. "Since 1976 -- 1977 really -- the performance of every single product out there has been upgraded," said McGuire. "Tampax, old staid Tampax who never did anything, they have a product they are calling superplus, which is 100 percent superabsorbent," she said.

Most of Rely's major competitors use a different material, a polyacrylate rayon fiber. (Tampax buys the fiber -- called Absorbit -- from a North Carolina company, as does Johnson & Johnson for its new o.b. tampon.)

As reported cases of toxic shock syndrome began to increase suddenly this year, and the Center for Disease Control began to zero in on the illness, the starting point was the question: What has changed in the environment of lifestyles of the young women who almost exclusively were its victims.

Most of the victims became ill within five days of beginning a menstrual period, although about 40 out of the 340 reported victims were men, the CDC reported in May. (The bacterium is also found on the skin and in the mouth, and can enter the bloodstream through sores or cuts there.)

But the link with menstruation was the first in the sequence of clues that led ultimately to tampons. Three more studies followed, based on interviews with 92 women, 91 of whom regularly used tampons.These findings were released in June.

In July and August, the CDC contacted 52 women who had been toxic shock victims and found that 50 regularly used one kind of tampon. Seventy percent of these women used Rely, indicating an unusually strong link between the P&G tampon and toxic shock syndrome. Among women generally who had not suffered toxic shock syndrome, only one in four used Rely, a CDC survey showed.

Investigators were confronted with four possibilities, said Dan. Either the women had changed, the tampons had changed, the infectious bacterium that caused toxic shock syndrome had changed or -- most likely -- some combination of these was at work.

"We knew tampons had changed," Dan said in an interview. He and other researchers now believe that the suspect bacterium, staphylococcus aureus, also has change recently, undergoing a mutation into a unique, particularly dangerous stain that doctors have not encountered before. It may produce a toxin (poison) or may carry the toxin with it in its genetic material.

The postwar development of antibiotics probably has created the conditions that make it possible for a new, mutant strain of infectious bacterium to emerge out of nowhere, Dan said. As the vulnerable staph bacteria are killed off by antibiotics, those that remain are tough customers, resistant to today's drugs.

'We've canvassed the whole country," said Dan. Most doctors and public health officials say the syndrome has shown up within the past year. "They've never seen anything like it, I think the disease is new."

What is there about tampons that provides the link with the disease? Dan says the CDC still is looking for an answer.

One theory is that the superabsorbent tampons provide a better breeding place for the staph bacterium.So far, though, that hasn't checked out, he said. "We've been examining the component parts of the tampons to see whether they could enhance bacterial growth, and we haven't found anything in that tampon (Rely) or any other that would back up the data implicating tampons," he said.

And it doesn't appear that Rely or other superabsorbent tampons are used longer than conventional brands, Dan said. Rely and its competitors are inserted with a plastic tube, and some experts wonder whether the tube may cause abrasions in a victim's vagina, permitting the bacterium to enter the blood stream. The CDC doesn't have sufficient medical data to prove this theory yet, Dan said.

There is no way of predicting when further surveys or research may solve the mystery of toxic shock syndrome, Dan said. In the meantime, the Food and Drug Administration is warning women not to use Rely, and if they continue to use another tampon, to alternate with a sanitary napkin. The govenment and the industry expect to publicize the symptons of the illness so that young women who come down with a sudden fever or vomiting will be alert to the possibility of toxic shock syndrome.

As for Procter & Gamble, it is facing an after-tax loss of $75 million, assuming Rely never comes back on the market, and a long struggle in the courts against women who suffered toxic shock syndrome, and the families of women who died from it.