This invention relates to muscle-strengthening agents comprising the hydrolysis products of whey proteins, wherein said agents are safe, highly transparent as water solutions, have reduced bitterness which does not limit flavor adjustments and are therefore readily suitable for product use, and have muscle-strengthening effects. The muscle-strengthening agents of the present invention comprise the whey protein hydrolysis products wherein the molecular weight range is 10kDa or smaller with the main peak in the 200Da~3kDa range, APL (average peptide length) is 2~8, free amino acid content is 20% or less of the total constituents, and antigenicity is 1/100,000 or less of that of beta-lactoglobulin.提供一種由具有肌肉增加效果之乳清蛋白質水解物構成之肌肉增強劑，係安全性優異且水溶液為高透明性，因為苦味少、無風味上限制，使用在製品時限制少。提供一種使用乳清蛋白質水解物之肌肉增強劑，該乳清蛋白質水解物係分子量分布為10kDa以下且主要峰部為200Da~3kDa、APL(平均胜肽鏈長)為2~8，相對於全部的構成成分，游離胺基酸含量為20%以下，抗原性為β-乳球蛋白的抗原性的1/100,000以下者。
