Mammary epithelial cells, harvested from either mammary tissue or milk from cows treated with growth hormone or atropine, were cultured in vitro. Protein synthesis in these cells was determined by measuring the incorporation of [3,4,5]-³H-leucine into protein secreted into the cell culture medium. Incorporation was measured between 0 and 11 h after addition of labelled leucine to the cell culture medium. No difference in incorporation was observed between epithelial cells isolated from milk compared to those from mammary tissue, nor was there any difference in incorporation in epithelial cells derived from the milk of control, growth hormone or atropine-treated cows.