Abstract: Novel constructs are provided for expression of physiologically active mammalian proteins in plant cells, either in culture or under cultivation. The constructs provide a promoter functional in a plant host, a structural gene coding for mammalian protein and a terminator functional in a plant host. The construct is introduced into a plant cell to become integrated into the plant genome for expression in the plant cells or plants. The plant cells may be harvested and the mammalian protein isolated in physiologically active form.