Effect of several hydrolytic reagents was ex-amined on the proteinaceous amino acid recovery from the residues after the ethanolic extraction of a mushroom, Pleurotus ostreatus . Over 85% of amino acid recovery was achiev-ed by 6 N HCl, 3 N p -toluenesulfonic acid, and 4 N methanesulfonic acid reagents, though the recovery was lower in 3 N mercaptoethanesulfonic acid. The highest recovery was obtained when 1, 500-volume to the sample weight of 6 N HCl was used. The other two reagents, p -tolueneand methane-sulfonic acids, were also applicable to the hydrolysis of mushroom proteins. The proteinaceous amino acid compositions were almost the same in every hydrolyses.