Owing to the high affinity and specificity of the avidin-biotin complex, this system has become a very useful tool in all fields of the biosciences. 1·2 The avidin-biotin complex has thus been used for isolation studies, 1 labeling and localization, 1 as well as for immunoassay techniques. 3"5 In all cases the principle involved is the binding of the biotin molecule to a compound whose isolation or localization is desired, while the avidin counterpart is affixed to a detectable probe (e.g., radioactive, fluorescent, or electron-dense label). For isolation purposes the avidin molecule is either immobilized or used to induce precipitation as a result of cross-linking via its four biotin-binding sites. Due to the versatility of this technique, it is not surprising that this system has also been introduced into the active field of liposome technology. 6

Among the various methods for probing cell surface membrane structure and function, liposomes have been employed to induce membrane fusion, for the incorporation of specific lipids and proteins into the cell membrane, and, more recently, for the targeted delivery of drugs and other substances into the cell. 7 •8 In all these studies, indirect methods were generally used to trace the fate of the liposome or to determine the extent of fulfillment of the required task. In our laboratory, we took a direct approach to this question by using the high-affinity avidin-biotin complex, an experimental system which we have developed for the localization and isolation of a wide variety of different classes of membrane-based components. 9 •10

For liposome technology, the approach involves the following steps:6 (1) biotin is covalently attached to the headgroups of appropriate lipids; (2) liposomes are prepared from characterized, biotinylated lipids; (3) the liposome preparation is interacted with viable cells; and ( 4) after a desired time interval the cells are washed and subjected to further interaction or analysis with avidin or a suitable avidin-conjugated probe.