4-Methylumbelliferyl (4-MU) oleate is a fluorogenic substrate for acid and alkaline lipases. It is cleaved by lipases, liberating 4-MU, which has an emission maximum at 445-454 nm. The excitation maximum for 4-MU is pH-dependent: 330, 370, and 385 nm at pH 4.6, 7.4, and 10.4, respectively.