Sterols are ubiquitous and essential constituents of all eukaryotic cells, found either esterified and stored in droplet form (l), or unesterified and integrated into bilayers (2). Sterols not only play a major structural role in membranes, but serve as precursors for steroid hormones and bile acids (3). The distribution of cholesterol within the cell and how it is apportioned properly are not only of fundamental interest but of cardinal medical importance. Despite intensive study (reviewed in 4” 7), the basic mechanisms of apportionment among cellular membranes of structural lipids in general and cholesterol in particular are still poorly understood. This brief review will focus on the utility of a particular probe, cholesterol oxidase.