4-Methylumbelliferyl-β-D-N,N',N''-triacetylchitotrioside is a fluorogenic substrate for chitinases and chitotriosidases. 4-Methylumbelliferyl-β-D-N,N',N''-triacetylchitotrioside is cleaved by chitinases and chitotriosidases to release the fluorescent moiety 4-methylumbelliferyl (4-MU). 4-MU fluorescence is pH-dependent with excitation maxima of 320 and 360 nm at low (1.97-6.72) and high (7.12-10.3) pH, respectively, and an emission maximum ranging from 445 to 455 nM, increasing as pH decreases. It has been used to screen plasma samples for reduced chitotriosidase activity that may be indicative of lysosomal storage disorders.