“It is a mechanistic use of animals that seems to perpetuate the notion of their being merely tools for human use rather than sentient creatures,” the Humane Society of the United States says in its position paper on the practice.

There are other concerns: that the animals could be harmed, that animal germs might contaminate the drug, that the milk or meat from genetically engineered drug-producing animals might enter the food supply or that the animals might escape and breed with others, spreading the gene, with unpredictable consequences.

But the F.D.A. approval could now encourage drug makers to consider this type of production.

The F.D.A.’s move “really takes away one of the biggest issues that have always been on the table, which is how do regulatory agencies view this kind of technology,” said Samir Singh, president of the American operations of Pharming, another company using such technology.

Pharming, which is based in the Netherlands, plans to apply this year for approval of a drug, produced in the milk of transgenic rabbits, to treat hereditary angioedema, a protein deficiency that can lead to dangerous swelling of tissues.

Another company, PharmAthene, is developing a treatment for nerve gas poisoning in the milk of transgenic goats.

Still, it could be difficult to persuade established manufacturers to depart from their existing methods, which have improved markedly since GTC first began its work.

Image A goat at GTC Biotherapeutics' farm. Credit... GTC Biotherapeutics

“I think we have very good ways of making therapeutic proteins today,” said Norbert Riedel, chief scientific officer at Baxter International, which makes proteins both from human plasma and in cell cultures grown in huge steel tanks.