Amphotericin B (AmB) binds to the cholesterol in lipoproteins, as determined by comigration in density gradient ultracentrifugation and changes in the circular dichroic spectrum. The saturation curve and Scatchard plots obtained with circular dichroism suggest that four to 10 cholesterol molecules in low-density lipoproteins bind to one molecule of AmB. AmB interacts more rapidly with low- and very-low-density lipoproteins than with high-density lipoproteins, but the circular dichroic spectrum of the complexed species is the same in all three cases. AmB also binds to other proteins in blood, but much higher concentrations of these proteins than of lipoproteins are needed for comparable binding. Interaction with lipoproteins stabilizes the antifungal activity of AmB. Interaction with lipoproteins and with much higher concentrations of other proteins in blood can also inhibit the effects of AmB on red blood cells, which contain cholesterol in their plasma membranes, but not the effects on Candida albicans, whose membranes contain ergosterol. An appropriate inference is that, when used clinically, AmB circulates in blood bound to lipoproteins and other proteins. The toxic and therapeutic effects of AmB in clinical situations are thus contingent on competitive interactions between sterol-containing cellular membranes of the host and the parasite and components of blood, such as lipoproteins and proteins.