In the present report we describe a simple and practical method to assess CETP activity in a defined system by use of microemulsions containing a fluorescent cholesteryl ester analog. The microemulsions are stable, simple to prepare, and can be made to defined composition. Initial transfer rates are easily determined by monitoring changes in fluorescence. We have used the fluorescent cholesteryl ester analog, cholesteryl 4,4-difluoro-5,7-dimethyl-4-boro-3 alpha, 4 alpha-diaza-3-indacenedodecanoate (BODIPY-CE), to demonstrate the utility of this assay. The assay takes advantage of the concentration-dependent self-quenching of BODIPY-CE, when this analog is incorporated into microemulsions. We have used this new assay to demonstrate fluorescent lipid transfer facilitated by rabbit and human d > 1.21 g/ml plasma fraction and recombinant human CETP. A known inhibitory monoclonal antibody (Mab) to human CETP blocked BODIPY-CE transfer in a dose-dependent manner. We have also used BODIPY-CE microemulsions to measure CETP activity in whole plasma.