—By an adaptation of the fluorometric method of Campbell and Moss (1961), the activity of α-naphthyl acid phosphatase was measured in individual neurons of monkey and human spinal cord and found to be many times higher in nerve cell bodies than in the surrounding neuropil. It was also measured in cerebellar cortex and found most concentrated in the granular (neuronal) layer. As this distribution is distinctive and paralleled by two other acid hydrolases, β-galactosidase and β-glucuronidase, it is considered to offer additional support for the lysosomal concept in nervous tissue and to indicate that nerve cell perikarya are much richer in lysosomes than are axons, dendrites or glial cells.