The carbohydrate composition was determined for ceramide hexosides isolated from brains of patients with Tay-Sachs disease and generalized gangliosidosis (hereby named GM1-gangliosidosis). Gray matter of patients with each disease showed a characteristic abnormal ceramide hexoside pattern. In Tay-Sachs gray matter, ceramide trihexoside is the major component, whereas ceramide tetrahexoside is barely detectable. In GM1-gangliosidosis, ceramide tetrahexoside is the major ceramide hexoside, while ceramide trihexoside is present only in small amount. These two major components have been characterized as the asialo derivatives of, respectively, the "Tay-Sachs ganglioside" (GM2-ganglioside) and the normal major monosialoganglioside (GM1-ganglioside). In both diseases, more than half the ceramide monohexoside of gray matter was glucocerebroside. Gray matter ceramide dihexoside, present in both diseases at higher than normal levels, was mostly ceramide lactoside, with possibly a small amount of ceramide digalactoside. Sulfatide contained only galactose. The abnormal ceramide hexoside pattern is limited to gray matter: white matter showed normal ceramide hexosides, i.e. a preponderance of monohexosides and sulfatide, with no detectable glucocerebroside.