A Florida company, AquaGene, has inserted genes into a common American freshwater species, tilapia, so that its liver secretes Human Coagulation Factor VII into its bloodstream.
The researchers have tested the substance on samples of blood taken from patients with haemophilia, but a long assessment road lies ahead before it can be certified as safe and effective for humans.
The fish approach is one of several avenues being explored by biotechnologists as they seek to derive therapeutic proteins to treat rare or catastrophic diseases.
Researchers are looking into genetically modified plants, chicken eggs and silkworm larvae, as well as cows and sheep, which produce the protein in their milk.
University of Southampton scientist Norman Maclean, who is working on the tilapia project, told the New Scientist that fish were of interest because there is no evidence that any disease can be transmitted from fish to humans.
In addition, transgenic fish are cheap and relatively easy to make and contain, whereas it can cost millions to produce genetically engineered farm animals and ensure they never have contact with wild species.
Human Factor VII can be purified directly from human blood, but there is a risk of transmitting diseases this way.
The only alternative to Human Factor VII, called NovoSeven, is derived from genetically modified hamster cells. The cost of a single injection can be as much as $10,000.
The report is carried in this Saturday's issue of New Scientist, a London-based weekly.