A post-coupling procedure for the quantitative measurement of N-acetyl-β-glucosaminidase activity in unfixed tissue sections of guinea-pig thyroid is described. The method depends on the cleaving of a naphthol AS-BI substrate and the insoluble reaction product is post-coupled with Fast Garnet GBC salt (in acetate buffer, pH 6.2) at 4° C. Even though this enzyme is localized in the lysosomes, an inert colloid stabiliser, polyvinyl alcohol (G18/140) is included in the reaction medium to allow the use of the optimal substrate concentration (0.5mg/ml) whilst employing a low concentration (5%) of ethylene glycol monomethyl ether. The high molecular weight (90.000) grade of polyvinyl alcohol used did not stabilize the lysosomal membrane, although a lower molecular weight (15 000) grade of polyvinyl alcohol (G04/140) may do. The eazyme activity was not affected by the metal ions Ca2+and Zn2+and was totally abolished by the specific inhibitor 2-acetamido-2-deoxy-d-gluconolacton