(FORTUNE Magazine) – CANCER is no longer a taboo subject at advertising agencies. Well before President Reagan's operation for cancer of the colon, Kellogg Co., the U.S. cereal king (1984 sales: $2.6 billion), launched a bold and controversial campaign positioning its All-Bran as a cancer preventive. ''At last, some news about cancer you can live with,'' proclaimed the Kellogg ad last fall. It was the first time a major food company had enlisted cancer to pitch a product. Now the tactic is spreading. This summer two sizable drug companies, American Home Products and Schering-Plough, are touting sunscreens and moisturizers as protection against skin cancer. In talking openly about cancer, Kellogg took a major risk. A 1983 National Cancer Institute study found that half the people surveyed believe that just about everything causes cancer, and they don't want to hear about it. To help dispel that fatalistic attitude, early last year the institute published a set of health guidelines. Among them: to reduce the risk of cancer caused by diet, eat high-fiber, low-fat foods -- fruits, vegetables, and grains. Figuring that many health-conscious Americans, particularly the baby-boomers, would respond to the message, Kellogg got institute approval to print the guidelines on its bran cereal boxes. ''We are endorsing a category of product rather than the brand,'' the institute's Paul Van Nevel insists. Still, the implication is clear: a bowl of All-Bran a day keeps cancer away. And so far the reception of the half-dozen bran products Kellogg now dishes out has been excellent, the company says. According to Alan Greditor, a security analyst at the Drexel Burnham Lambert investment firm, putting cancer on the breakfast table is part of Kellogg's successful drive to reverse a five-year erosion of market share. About 41% of all the cold cereals consumed in U.S. households last year came from Kellogg pantries, compared with 36.5% & in mid-1983. Greditor estimates that of the $1.5 billion Kellogg racked up in cold cereal revenues, some $250 million came from bran sales. Sales of all Kellogg brands rose 10% last year. Competitor General Mills will soon launch a new Fiber One cereal, and General Foods is adding two more Fruit & Fibre varieties, but neither company has yet promoted the anticancer connection. Drug companies haven't been shy, however. Since the late 1970s they have warned on many suntan-lotion packages that overexposure to the sun could lead to cancer. This summer American Home began peddling its Youth Garde moisturizer as a way ''to protect against skin cancer.'' Schering-Plough introduced its Shade sunscreen with a model who announces during the TV commercial that she has had the disease and recovered. Madison Avenue expects that a lot more advertisers will soon be making claims about lowering cancer risks. Two days after the announcement of the President's colon cancer, more than 2,500 people had called the National Cancer Institute for advice on how to avoid the disease. Many will doubtless heed the counsel that the President's chief surgeon, Dr. Steven Rosenberg, gave his famous patient. Rosenberg's prescription: eat high-fiber foods.