Responding to consumer concerns about genetic engineering and food safety, those gallons will soon be free of Posilac, a controversial synthetic growth hormone used to boost milk production.

Last month, the company committed to making 100 percent of the milk supply for its more than 5,600 American locations free of the synthetic bovine growth hormone -- officially known as recombinant bovine somatotropin (rbST) -- by the end of the year.

Grocery retailer Krogers has also said it would only sell rbST-free by early 2008.

"This is a very clear message to the dairy industry that consumers don't like having their cows treated with bovine growth hormone," said Marion Nestle, a professor in the Department of Nutrition and Food Studies at New York University and author of the book "What To Eat".

Posilac, sold by Monsanto Co., is the genetically engineered version of a hormone produced by lactating cows and found in all milk. The two hormones are chemically very similar, and the Food and Drug Administration (FDA) says there is no difference between milk from treated and untreated cows.

The synthetic hormone has been controversial since it received FDA approval in 1993, amid scientific and consumer criticism and heavy lobbying by Monsanto. Critics argue that rbST was never properly cleared of safety concerns, and Canada, Australia, New Zealand, Japan and the European Union all refused to approve Posilac because of animal health concerns.

Today, about 30 percent of dairy cows in the United States are given biweekly injections of Posilac, which can increase their milk production by 10-20 percent. Monsanto said Starbucks's decision not to use milk produced with rbST will have negative consequences for farmers who use Posilac to increase profits and reduce resource consumption by getting more milk per cow.

"There's no difference in the milk, so a policy that discriminates against farmers who produce the same milk is unfortunate when it has a negative impact on the bottom line," said Monsanto spokesman Andrew Burchett.

The hormone itself likely poses no health risk. Both the natural and synthetic versions are specific to cows and structured differently than human growth hormones. As protein hormones, they are largely inactivated by stomach acid and further broken down by digestive enzymes in the intestines.

Recombinant bovine somatotropin does pose a risk to cows, however -- those given the growth hormone injections also have a 25-percent higher risk of mastitis, an udder infection, because of the increased milk production. Treated cows have also shown higher risk of infertility and lameness in studies.

More worrying is the possibility of a human health risk from the insulin-like growth factor IGF-1, a hormone found in slightly higher quantities in milk from cows treated with rbST. Unlike growth hormones, the human and bovine forms of IGF-1 are identical, and studies have shown a correlation between elevated levels and modest increases in rates of prostate and breast cancer.

There is no clear connection to the consumption of milk produced with rbST, however, and other factors like weight, physical fitness and genetics can affect IGF-1 levels in the blood. Also, because the hormone is a protein, much of it may not survive digestion. "I just don't think there's enough data to say anything about it one way or another," said Nestle, but IGF-1 remains a safety concern.

Consumer worry about rbST ties in with concerns about genetically engineered food, which does not have to be labelled. When the FDA approved Posilac, it also decided that labelling the milk produced using the hormone wasn't necessary.

(Nestle was a member of the four-person committee of consumer representatives who recommended mandatory labeling to the FDA.) Monsanto argued that labeling would mislead consumers into believing that there was a difference between milk with and without rbST.

Critics disagree. "If they choose not to use something, and that distinguishes them from their competitors, they should be able to say so," said Patty Lovera, assistant director of Food and Water Watch (http://www.foodandwaterwatch.org/).

Starbucks became the target of Food and Water Watch's "Hold the Hormones" campaign a year and a half ago, and 72 percent of their dairy was rbST-free as of last month's commitment to a total phase out.

Author Nestle also argues that even if rbST is safe, that doesn't mean consumers are comfortable having it in their milk. "It was kind of shoved down the throats of the American people, much against their better judgement, for reasons of science that have nothing to do with policy or the way people feel about these things," she said.

The government is not responsive to concerns about rbST, Lovera said, so change will come from consumers and organized action aimed at large, influential companies like Starbucks.

"The progress on this issue has come from the marketplace," she said. "We thought if we can get another big company to switch, that's going to convince more dairies to get rid of this hormone."

Consumer feelings about artificial growth hormones is so strong that, soon, only rbST-free milk will be sold in the United States, said Ronnie Cummins, national director of the Organic Consumers Association. The power of consumer pressure to change market dynamics is encouraging, he said, despite the FDA's refusal to change its stance on rbST.

Cummins adds that, overall, the tide may have turned on food safety. "Once you realize what's going on with modern food and farming systems and you make a decision that you're going to try to purchase alternatives to protect your health and your family's health," he said, "you're going to keep going."