This chapter describes in detail a practical procedure for the preparation of 
radiolabeled galactocerebroside and its use in the assay of galactocerebrosidase 
(GalCase), the enzyme deficient in globoid cell leukodystrophy (Krabbe disease). 
The reference range for leukocytes and fibroblasts is 0.9-4.4 and 8-36 nmoles 
substrate hydrolyzed per hour per milligram of protein, respectively. Because of 
its low abundance this enzyme is difficult to assay in certain situations, such 
as prenatal diagnosis by chorionic villus sampling. To obviate this a modified 
assay is used where only the radiolabeled substrate is included in the 
incubation. This provides a clear separation between affected samples and 
unaffected controls. The methods detailed here should be reproducible in any 
laboratory.