The Kellogg Company said yesterday that it would phase out advertising its products to children under age 12 unless the foods meet specific nutrition guidelines for calories, sugar, fat and sodium.

Kellogg also announced that it would stop using licensed characters or branded toys to promote foods unless the products meet the nutrition guidelines.

The voluntary changes, which will be put in place over the next year and a half, will apply to about half of the products that Kellogg currently markets to children worldwide, including Froot Loops and Apple Jacks cereals and Pop-Tarts.

Frosted Flakes, for example and Rice Krispies with Real Strawberries will still make the nutritional cut, though regular Rice Krispies will not (too much salt).

The president and chief executive of Kellogg, David Mackay, said that the products that did not meet the guidelines would either be reformulated so that they did, or no longer be advertised to children.

“It is a big change,” Mr. Mackay said. “Where we can make the changes without negatively impacting the taste of the product, we will.”

If the product cannot be reformulated, Mr. Mackey said, the company will either market it to an older audience or stop advertising it.

The policy changes come 16 months after Kellogg and Viacom, the parent company of Nickelodeon, were threatened with a lawsuit over their advertising to children by two advocacy groups, the Center for Science in the Public Interest and the Campaign for a Commercial-Free Childhood, and two Massachusetts parents.

Because of the changes by Kellogg, the groups said that they would not proceed with the lawsuit against the company. Viacom had not negotiated with the groups and was not part of today’s announcement; the groups said that they had not determined if they would proceed with legal action against the broadcaster.

“Kellogg’s position has really evolved over those months from pretty much ‘no way’ to acceptance of some nutrient criteria,” said Michael F. Jacobson, executive director of the Center for Science in the Public Interest. He said he hoped the Kellogg announcement would lead its competitors to adopt even tougher standards for food advertising to children.

Susan Linn, the co-founder of the Campaign for a Commercial-Free Childhood, said that Kellogg’s decision to stop using licensed characters on sugary food was particularly significant. “Until now the industry has absolutely dug in their heels,” Ms. Linn said.

In the last several years, health officials have repeatedly warned that the steady stream of food ads aimed at children is contributing to the number of overweight or obese children, which has soared over the last several decades.

Some countries have banned advertising of nutritionally questionable food to children altogether, and some members of Congress have suggested that federal regulation may be needed in the United States, too. The food industry has promised to bolster its own self-regulation.

Last November, for instance, 10 of the largest food and beverage companies, including McDonald’s, General Mills and Kellogg, vowed that at least half of their advertising directed at children under 12 would promote healthier foods or encourage active lifestyles.

The companies also agreed not to advertise in elementary schools and to reduce the use of licensed characters to promote food. Those companies are expected to complete individual plans for how they will address the guidelines in the next 60 days or so.

But like Kellogg, a few companies have already unveiled tougher standards for advertising to children. Last October, for instance, Walt Disney said that it would allow its characters to be used in food advertising only if the products complied with nutritional standards.

And in 2005, Kraft Foods announced that it would stop advertising products to children under 12 that did not meet specific nutrition guidelines.

Under Kellogg’s new guidelines, food advertised on television, radio, Web sites and in print that have an audience that is 50 percent or more of children under 12 will have to meet the new nutrition standards. Kellogg already had a policy of not aiming advertising at children younger than 6, so the new guidelines apply to children 6 through 11.

Kellogg officials said that about 27 percent of its advertising budget in the United States aims at that age group. They declined to give the dollar value of that budget.

Under the new standards, one serving of food must have no more than 200 calories, no trans fat, no more than 2 grams of saturated fat, no more than 230 milligrams of sodium (except for Eggo frozen waffles) and no more than 12 grams of sugar.

Cocoa Krispies cereal would not qualify because one serving has 14 grams of sugar. But Kellogg could still advertise Frosted Flakes to children because it has 11 grams of sugar. Shrek cereal does not meet the criteria because it has 15 grams of sugar a serving and uses a licensed character.

In a related initiative, Kellogg said it would introduce Nutrition at a Glance labels on the top right corner of cereal boxes this year.

Already introduced in Europe and Australia, the new labels will take information from the Nutrition Facts panel on the side of the boxes, which are mandated by the federal government, and highlight important parts on the front of the box.

The new labels will show consumers the percentage of calories, total fat and sodium in a single serving, based on a 2,000-calorie daily diet, and will also display grams of sugar and nutrients like fiber and calcium.