SummaryTwo methods of microdetermination, enzymatic cycling and microscopic fluorometry, have been applied for the diagnosis of lysosomal diseases. A new assay method of galactocerebrosidase was developed by using the NAD cycling procedure. It was reproducible and even more sensitive than the radioassay method currently in use, and the enzyme deficiency was confirmed in the tissues from two patients and a fetus with Krabbe's disease. A microassay method of lysosomal enzymes on single fibroblasts was also established by microscopic fluorometry. With this technique a successful result of prenatal diagnosis was reported on a high risk fetus for GM1‐gangliosidosis. The basic and clinical significance of these micromethods was discusse