The developmental patterns of four lysosomal enzymes have been investigated in liver, kidney, lung, heart, spleen, muscle and brain tissues of human fetuses at varius gestational ages. The largest increment in the activity of all four enzymes, namely acid alpha-glucosidase, alpha-galactosidase, beta-galactosidase and acid phosphatase had been observed in kidney with a 6- to 12-fold increase between the second and third trimester of gestation. The activity of all liver and spleen enzymes also increased considerably during these periods. In muscle, however, only alpha-glucosidase and acid phosphatase showed an increase in the activity, and in lung, acid phosphatase and beta-galactosidase. Most of brain and heart enzymes, except acid phosphatase, did not change significantly during the observation period. The activities of these lysosomal enzymes were also measured in tissues of a normal adult individual, and aspects of the neonatal and postnatal development of these enzymes were discussed.