4-Methylumbelliferyl phosphate serves as a fluorogenic substrate for calmodulin-dependent protein phosphatase and in kinetic studies of alkaline phosphatase. It is a substrate for alkaline phosphatase in enzyme-linked immunosorbent assay (ELISA) procedures. 4-Methylumbelliferyl phosphate is seven times more sensitive than phenolphthalein monophosphate and 8-13 times more sensitive than with p-nitrophenyl phosphate when used in enzyme immunoassays for the detection of antibodies to human immunodeficiency viruses.