GM2 gangliosidoses, including Tay-Sachs disease, Sandhoff disease and the AB variant, comprise deficiencies of beta-hexosaminidase isozymes and GM2 ganglioside activator protein associated with accumulation of GM2 ganglioside (GM2) in lysosomes and neurosomatic clinical manifestations. A simple assay system for intracellular quantification of GM2 is required to evaluate the therapeutic effects on GM2-gangliosidoses.We newly established a cell-ELISA system involving anti-GM2 monoclonal antibodies for measuring GM2 storage in fibroblasts from Tay-Sachs and Sandhoff disease patients.We succeeded in detecting the corrective effect of enzyme replacement on elimination of GM2 in the cells with this ELISA system.This simple and sensitive system should be useful as additional diagnosis tool as well as therapeutic evaluation of GM2 gangliosidoses.