PROBLEM TO BE SOLVED: To provide an agent for increasing a protein content in milk of a lactating cow. SOLUTION: 5-ALA which is a kind of amino acid, or a derivative thereof, or ALA which is a salt thereof is mixed with a feed and administered to a lactating cow without any treatment such as coating. There was almost no change in milk yield compared to milk collected from the dairy cow herd, but milk with a significantly higher proportion of protein, especially casein (%), was obtained. [Selection diagram] None【課題】泌乳牛の乳におけるタンパク質含量の増加剤を提供する。【解決手段】アミノ酸の一種である５－ＡＬＡ、若しくはその誘導体又はそれらの塩であるＡＬＡ類を、コーティング等の処理を施すことなく、飼料に混合して泌乳牛に投与したところ、従来の飼料を摂取した乳牛群から採取した乳と比較して、泌乳量にはほぼ変化がなかったが、タンパク質、とりわけカゼインの割合（％）が有意に多い乳を得た。【選択図】なし
