This paper describes the possible utility of plasma lipoproteins for the site-specific delivery of diagnostic agents. The class of lipoproteins known as chylomicrons was selected for this preliminary study, since they are known to be rapidly metabolized and taken up by the liver. Cholesteryl iopanoate (II), an iodinated analogue of a normal constituent of the hydrophobic core of chylomicrons, was synthesized from cholesterol and iopanoic acid (I) and subsequently radiolabeled with ioidine-125. Whereas intravenous administration of II in physiological saline resulted in the appearance of approximately 31% of the dose in the liver at 0.5 hr, prior incorporation of II into chylomicrons resulted in an almost threefold (87%) increase in the liver accumulation of II in the same time period. A more gradual appearance of II in steroid-secreting tissues was consistent with the association of II with high-density lipoproteins following administration.