The free cholesterol content of cells can be monitored by the intensity of fluorescence emissions from the polyene antibiotic filipin. In a previous study (Hassall: Cytometry 13:381-388, 1992) using THP-1 macrophages, a decrease in filipin fluorescence in response to increasing concentrations of modified lipoprotein was observed, suggesting a reduction in the free cholesterol content of the cells. In this study, THP-1 macrophages were treated with a number of agents known to modulate cholesterol biosynthesis and cholesterol esterification. Changes in filipin fluorescence emissions were measured by flow cytometry, and correlated with changes in cholesterol biosynthesis measured by incorporation of [14C]acetate into cholesterol. A correlation between decreases in filipin fluorescence and reductions in cholesterol biosynthesis was apparent, even when cholesterol esterification was inhibited. These results suggest that the decreases in filipin fluorescence observed may be due, at least in part, to reduction in cholesterol biosynthesis.