Effects of methionine dose and its ratio to lysine on α_s1 casein gene expression were evaluated in cultured bovine mammary epithelial cells. Methionine was added at levels of 0-100 μg/ml and ratio of lysine to methionine was set at 1-4.5:1. Expression of α_s1 casein gene was enhanced by addition of methionine up to 60 μg/ml, and then leveled off. There was an increasing trend in α_s1 casein expression with the increasing ratio of lysine to methionine and the highest level was at 3.5:1. These results indicated that adequate quantities and ratio of amino acids are required to promote milk protein synthesis.