To express recombinant proteins in the spontaneously immortalized porcine mammary epithelial cell line (SI-PMEC) currently established in our laboratory, a chemically synthesized DNA fragment encoding the anticoagulant hirudin was used to construct a mammalian expression vector under the control of the goat β-casein regulatory sequence. The vector, named pGB562/Hi, was transfected into the SI-PMEC cells to yield pGB562/Hi/SI-PMEC. The pGB562/Hi/SI-PMEC cells expressed recombinant hirudin only when they were differentiated into functional structures by growth on a Matrigel-coated petri dish supplemented with the lactogenic hormone prolactin. The differentiated pGB562/Hi/SI-PMEC cells produced about 0.5-0.6 μg of recombinant hirudin/mg of total cellular protein. These results suggest that the established SI-PMEC cells have pharmaceutical potential to inducibly express bioactive heterogeneous proteins.