A transgenic, non-human mammalian animal is capable of expressing a heterologous gene for human or other recombinant physiologically functional fibrinogen holoprotein or individual subunit chain polypeptides thereofora modified or fusion fibrinogen in mammary glands of the animals and secreting the expressed product into a body fluid. Methodol ogy employing such a mammal yields recombinant physiologically functional fibrinogens, subunit chain polypeptides thereof, and modified or fusion fibrinogens
