4-Methylumbelliferyl N-acetyl-a-D-neuraminic acid ammonium is an acetylated form of the natural substrate for alpha-N-acetylneuraminic acid hydroxylase, which is a key enzyme in the synthesis of sialic acids. 4-Methylumbelliferyl N-acetyl-a-D-neuraminic acid ammonium is used as a chromogenic or fluorogenic substrate for detecting alpha-N-acetylneuraminic acid hydroxylase activity in the cell. It is also used as a bioluminescence substrate and chemiluminescent substrate in immunoassays.