Riboflavin-5'-monobutyrate (R-MB, mp 244-245°) was synthesized from riboflavin and butyric anhydride (yield : 10%) as well as from riboflavin-2', 3', 4', 5'-tetrabutyrate and hydrazine hydrate (yield : 44%). Riboflavin-5'-monopalmitate (R-MP, mp 228-230°) was also prepared from riboflavin and palmitic anhydride (yield : 9.8%). The absorption, fluorescence, and infrared spectra, and Rf values of R-MB and R-MP are given. R-MP was hardly hydrolyzed by pure hog hepatic carboxylesterase. However, R-MB was readily hydrolyzed to riboflavin by pure hog hepatic carboxylesterase, though not by pure pancreatic lipase (triacylglycerol lipase), acetylcholinesterase, or cholinesterase. The activity of carboxylesterase could therefore be estimated by fluorometric determination of riboflavin hydrolyzed from R-MB. The optimum incubation conditions for the assay of carboxylesterase with R-MB were pH 7, 37°, 15 min. Studies on the rat organs and liver subcellular distributions of carboxylesterase showed that the enzyme is localized in the small intestine, kidneys, lungs, heart, and blood, and high carboxylesterase activity was demonstrated in the microsomal fraction in rat liver. R-MB was a rather specific substrate for carboxylesterase, permitting selective assay of this enzyme activity.