Two major and several minor isozymes of hexosaminidase (Hex) exist in human tissues. In Tay-Sachs disease Hex A is missing but the activity of Hex B is increased. In Sandhoff's disease, both Hex A and Hex B are lacking; the residual activity represents increased amounts of a normal minor isozyme, Hex S. A fourth isozyme, Hex C, has an electrophoretic mobility very similar to that of Hex S, but seems to be unrelated genetically and structurally to A, B and S. The model which seems best to explain the relationship between Hex A, B and S is one in which they are polymers comprised of two different polypeptides which we have designated the α and β chains1. We have proposed that Hex B is a β-chain homopolymer, Hex A an α and β-chain heteropolymer and Hex S an α-chain homopolymer2. We now report that all interconversions between Hex A, B and S that are predicted by this subunit model can be performed.