(1) The synthesis of 2-methoxy-4-(2'-nitrovinyl)-phenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside (MNP-GlcNAc) and 2-methoxy-4-(2'nitrovinyl)-phenyl beta-D-galactopyranoside (MNP-Gal) as substrates for the assay of NAG and beta-d-galactosidase are described. (2) beta-Glycosidase activities were determined in random urine samples from normal males and females aged between 12 and 87 years and patients with renal disease. (3) Both the MNP N-acetylglucosaminide and MNP galactoside were stable indefinitely, if stored in the solid state at 4 degree C in the dark. (4) The effect of urinary inhibitors was minimized by diluting the urine in the assay procedure. A simple assay procedure has been developed using MNP substrates. A good correlation was found with established assays using 4-methylumbelliferyl and p-nitrophenyl glycosides. (5) The assay was readily automated and a good correlation was found between the automated and manual methods. (6) The assay of urinary glycosidase activity with MNP substrates is simple to perform and has been used successfully in the clinical chemistry laboratory.