The automated fluorimetric assay of N-acetyl-beta-D-glucosaminidase (NAG) has proved to be of value in the detection of rejection in transplant patients and in monitoring the course of renal disease. The urine can be diluted prior to assay, due to the sensitivity of the fluorimetric method and this reduces the effect of endogenous inhibitors. Time consuming dialysis or gel filtration steps are therefore unnecessary. Results are available to the physician within hours of the collection of the urine sample, since factoring the enzyme activity by the creatinine concentration avoids the necessity of collecting twenty-four hour or timed urine samples. Over one hundred samples can be assayed in a day using the automated procedure. The excretion of NAG in normal individuals varies with age and the activity found in pathological samples should always be compared with age-matched controls. The development of a new series of colorimetric substrates has allowed the production of a simple 'Dipstick' which should be of value in the screening of at-risk populations for renal disease. The diagnostic potential of the estimation of urinary NAG activity is enhanced by the separation of its isoenzymic forms by ion-exchange chromatography on DEAE-cellulose.