PORTLAND – Oakhurst Dairy Inc. is being sued by Monsanto Co., which alleges that Oakhurst’s marketing campaign that touts its milk as being free of artificial growth hormones is misleading.

The suit, filed in U.S. District Court in Boston, demands that Oakhurst stop advertising that it doesn’t sell milk from hormone-treated cows. It also asks that the dairy stop putting labels on its milk containers reading “Our Farmers’ Pledge: No Artificial Growth Hormones.”

Monsanto officials said Oakhurst’s ads and labels are deceptive and disparage Monsanto’s products with the inference that milk from untreated cows is better than milk from hormone-treated cows.

“We believe Oakhurst labels deceive consumers; they’re marketing a perception that one milk product is safer or of higher quality than other milk,” said Jennifer Garrett, director of technical services for Monsanto’s dairy business. “Numerous scientific and regulatory reviews throughout the world demonstrate that that’s unfounded. The milk is the same, and the amount of protein, fats, nutrients, etc. are all the same.”

Oakhurst President Stanley Bennett II said his dairy sells milk without artificial growth hormones because of consumer demands. Oakhurst about five years ago began buying milk only from farms that pledge in writing that they won’t use artificial hormones.

“On principle, it’s also a question of free speech,” Bennett said. “The world seems a little bit discombobulated when somebody attempts to prohibit you from trying to do the right thing.”

Artificial growth hormone is a genetically engineered veterinary drug given to cows to increase milk production. Another name for the drug is recombinant bovine somatotropin, or rBST.

Many people oppose the use of rBST, believing it is linked to breast cancer and premature puberty in children. But Monsanto and others argue that no such link exists.

Canada and the European Union have banned the use of the hormone, but the Food and Drug Administration has approved it for use in the United States.

Monsanto, which is based in St. Louis and is the leading producer of rBST, had revenues of $4.7 billion in 2002. Oakhurst, based in Portland, had sales of $185 million, according to Bennett.

Monsanto spokesman Lee Quarles said Monsanto has not filed similar lawsuits against other dairies, but wouldn’t say whether more were planned. Monsanto filed similar suits against two dairies in Illinois about 10 years ago, and both were settled out of court under confidential terms, he said.

The suit against Oakhurst claims unfair competition, unfair business practices and interference with advantageous business relationships. According to the suit, the business relationships between Monsanto and dairy producers who use the artificial growth hormone have suffered because the farmers will stop using the treatments.

Bennett said his company makes no claims on the science involved with growth hormones.

“We’re in the business of marketing milk, not Monsanto’s drugs,” he said.

Earlier this year, Maine Attorney General Steven Rowe rejected a request from Monsanto that Maine abandon its Quality Trademark Seal program that indicates when milk is free of artificial growth hormones.

Monsanto argued that the seal, which was adopted in 1994, misleads consumers into thinking that hormone-free milk is superior to milk using an artificial growth hormone.

According to Garrett, an independent market study conducted in Massachusetts shopping malls showed that more than two-thirds of the 300 people surveyed thought that milk with the Oakhurst labels were healthier to drink than milk without such labels. Sixty percent of those surveyed thought that Oakhurst milk was safer to drink, Garrett said.