This paper describes a comparison of the recently developed substrate 2-methoxy-4-(2'-nitrovinyl)-phenyl-2-acetamido-2-deoxy-beta-D-gluc opyranoside (MNP-GlcNAc) and the corresponding 4-methylumbelliferyl substrate (4-MU-GlcNAc) for the determination of urinary NAG. A good correlation (r = 0.977) was found between NAG activities in 366 urine samples from renal transplant patients determined by either the fluorimetric method or the colorimetric procedure. The colorimetric method may be used with confidence as an alternative to the fluorimetric assay and does have the advantage that colorimetry is widely used in clinical chemistry laboratories. Sample blanks are not normally required unless there is blood contamination in the urine which can easily be identified visually. The incorporation of the MNP-GlcNAc substrate into a kit suitable for the assay of urinary NAG using a simple battery-operated miniphotometer is also described. The presence of elevated NAG levels in urine can be detected rapidly and this procedure is suitable for routine monitoring of renal patients in the laboratory and may prove to be suitable for use by non-laboratory personnel in clinics or on the ward.