Histochemical examination of the activity of naphthylamidase (LNAse) in the cerebellar cortex of 70 human autopsies consistantly revealed a marked activity mainly in the internal granular layer with pH optimum of 5.8. Slight enzyme activity was also localized in sites corresponding to lipofuscin deposits and areas of acid phosphatase activity in the Bergmann glial cells, Purkinje cells and in perivascular cells. The histochemical findings support the LNAse reaction as a lysosome marker. Differences in localization of LNAse and acid phosphatase could possibly be due to prior release of the latter enzyme from the internal granular layer. Significant correlation between demonstrable loss of granule cell nuclei (the so-called acute, selective necrosis of the granular layer) and low pH of the cerebellar tissue could be demonstrated in 21 cases. The present findings support the hypothesis that an enzymatic disintegration of the granule cells takes place in postmortem cerebella with low pH simulating a necrotic vital phenomenon.