—A longitudinal study of the maturation of mouse cerebral lysosomal enzymes has been completed. Activity of the enzymes, acid phosphatase (I.U.B., β-glucuronidase (I.U.B. and β-acetylglucosaminidase (I.U.B. was assayed spectrofluorimetrically on portions of supernatant from 0.25 M sucrose homogenates spun at 6 x 103 -min. Activities were obtained in native (free) and Triton X-100 activated samples (total).

The neonatal period was characterized by relatively low free and high total acid phosphatase activities. An abrupt rise in free activity occurred during the period 10–20 days. Discontinuous anion exchange DEAE cellulose chromatography (0.01 m-tris–maleate, pH 6.3) with elution by ascending molarities of NaCl of the Triton X-100 activated supernatant revealed three major peaks in the adult. A fourth peak, designated as fraction II (‘maturation fraction’) occurred only during the neonatal period, a time also characterized by increased specific activity of fraction I, with no change in fraction IV. The chromatographic fractions were further characterized by optimal pH, ascorbate, fluoride, Cu2+ and Fe2+ ions.

The maturation profiles of total, β-glucuronidase and total, β-acetylglucosaminidase differed from each other, and from that of total acid phosphatase. Comparable differences existed in the profiles of the free activities, and the ratio of free:total activity differed for each enzyme at any selected time especially during the neonatal period. These findings are are discussed with reference to the maturation of isoenzyme fractions with age, and suggest that the changes in structure-linked organization of individual lysosomal hydrolases are functions of heterogeneity in enzyme complement of individual lysosomes.