Similarly, the use in milk production of bovine somatotropin, a natural cow hormone, yields milk that is chemically indistinguishable from milk produced by untreated cows.

Store owners said sales of milk and cheese from untreated cows were soaring.

Dr. Michael Pollack, who was involved with both studies, noted that the difference between the IGF-1 in milk from untreated cows and treated cows is relatively small.

The missing link is whether milk from rbGH-treated cows translates into higher IFG-1 levels in humans than milk from untreated cows.

Animal-rights activists say cattle given the drug tend to get more udder infections than untreated cows.

They say milk produced by cows that receive it is identical to milk from untreated cows.

The F.D.A. has said that milk from cows treated with the hormone is indistinguishable from milk from untreated cows.

Still, Dean Foods says only 10 of its 100 milk processing plants around the country offer milk from untreated cows.

Instead, the agency said that dairy companies that use milk from untreated cows could put labels on their products as long as the labels did not mislead consumers.

Such unqualified statements may imply that milk from untreated cows is safer or of higher quality than milk from treated cows.