Abstract: The present invention relates to a bovine immortalized mammary epithelial cell line which has normal physiological responses in that it produces milk constituents which comprises .alpha.- and .beta.-casein and lactose. There is provided, using the cell line of the present invention a method of `in vitro` studying lactation. There is provided a method of `in vitro` screening for gene expression of DNA constructs for transgenic cows, since the cell line of the present invention is a bovine one. The cell line can be further used in a method for indefinitely expressing foreign genes. The cell line of the present invention has been deposited at the ATCC under the accession number CRL 10274.