Abstract Estrogen replacement therapy is widely used to combat symptoms of menopause, but factors influencing the transport of estrogen in plasma and its cellular uptake have been explored only to a limited extent. In this study, labeled estradiol-17β (E 2) was compared with its hydrophobic derivative (estradiol-3,17-diacetate, E 2AA) in terms of (1) distribution within various lipoproteins and lipoprotein-free fractions of human plasma by ultracentrifugation and (2) uptake by human endothelial cells. Although added E 2 was predominantly bound to HDL and lipoprotein-free fractions, E 2AA was associated to some degree with VLDL and LDL but was still present in significant amounts in HDL and lipoprotein-free fractions. Significant associations of E 2 with lipoproteins were also confirmed by polyacrylamide gel electrophoretic separation of E 2-labeled plasma. In normal plasma, however, < 10% of E 2 was associated with lipoproteins when measured by radioimmunoassay. Incubation of E 2AA and E 2 with human dermal capillary endothelial cells showed similar uptake by 24 hours. A significant portion of E 2AA was hydrolyzed to estradiol-17-onoacetate both in plasma and in cells. The addition of HDL and LDL to medium containing lipoprotein-deficient serum significantly reduced labeled E 2 and E 2AA uptake, respectively, by endothelial cells. These studies suggest that (1) although the association of E 2 with HDL under normal conditions may be small, it increases significantly with higher estrogen concentrations of E 2; (2) the hydrophobic ester of E 2 shows increased binding to VLDL and LDL; and (3) this approach may be useful in manipulating the delivery of selected E 2 derivatives to cells.