Total N-acetyl-beta-D-glucosaminidase (NAG) and NAG-B isoenzyme determination are determined in order to investigate renal tubular function. Here, we propose a semi-automated adaptation of Sandman's manual method for the Monarch centrifugal analyzer. Fluorescence of the released 4-methylumbelliferone is automatically read and results are directly expressed in nanokatal/l. Mean (1 SD) values expressed in nanokatal/millimole creatinine in urine from healthy female (n = 30) and male (n = 30) subjects were 8.2 (4.0) and 7.8 (2.9) for total NAG and 1.9 (1.4) and 1.5 (0.7) for the NAG-B isoenzyme activity. The assay of six patients with various renal disorders shows definite increase in total NAG and NAG-B isoenzyme activities.