Supernatant Protein Factor (SPF), a protein that enhances the activities of microsomal squalene epoxidase and 2,3-oxidosqualene-lanosterol cyclase, has been labeled either by acylation with N-succinimidyl[2,3-3H]propionate or by reductive methylation with [14C]-formaldehyde and sodium cyanoborohydride. Labeled SPF preparations, containing 1 to 2 modified lysine residues/molecule of protein which retained full biological activity, were found to bind only weakly to microsomes under a variety of experimental conditions as determined by sucrose density gradient centrifugation. No interaction between SPF and either squalene or squalene-2,3-oxide could be demonstrated by gel filtration. On the other hand, SPF was shown to bind tightly to vesicles of anionic phospholipids (phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, and phosphatidic acid) but not to vesicles of phosphatidylcholine or phosphatidylethanolamine. The capacity of the anionic phospholipids to bind to SPF parallels their ability to enhance the stimulatory activity of SPF. These observations are inconsistent with the designation of proteins of this type as "sterol carrier proteins."