Summary: Gaucher disease, the most common sphingolipidosis, is caused by a decreased activity of glucosylceramide β-glucosidase, resulting in the accumulation of glucosylceramide in macrophage-derived cells known as Gaucher cells. Much of the storage material is thought to originate from the turnover of cell membranes, such as phagocytosed red and white blood cells. In this study, an in vitro model of Gaucher disease was developed by treating the murine macrophage cell line J774 with a specific inhibitor of glucosylceramide β-glucosidase, conduritol B-epoxide, and feeding red blood cell ghosts, in order to mimic the disease state. It was found in this model system that glucosylceramide β-glucosidase activity could be reduced to about 11–15! of the normal control level before increased storage of glucosylceramide occurred. This in vitro system allows insight into the correlation between enzyme activity and lipid storage as predicted by the theory of residual enzyme activity that was proposed by Conzelmann and Sandhoff.