The first step in the fusion of two phospholipid membranes culminates in the aggregation of the two lipid bilayers. We have used a custom-built fluorimeter to detect multilamellar vesicles (liposomes) containing the fluorescent dye, 6-carboxyfluorescein (6-CF), bound to a planar lipid bilayer (BLM). Liposomes were added to one side of the BLM, and unbound vesicles were perfused out. This left a residual fluorescence from the BLM, but only when the membranes contained anionic lipids, and then only when millimolar levels of calcium were present. This residual fluorescence was consistently detected only when calcium was included in the buffer during the perfusion. This residual fluorescence originated from liposomes bound to the BLM. Breaking the BLM or lysing the adsorbed vesicles with distilled water abolished it. free 6-CF and/or calcium in the absence of liposomes resulted in no residual fluorescence. No residual fluorescence was detected when both the liposomes and the BLM were composed entirely of zwitterionic lipids. This was found to result from the insensitivity of the fluorimeter to a small number of liposomes adsorbed to the BLM. For this system, we conclude that calcium is necessary for both the initiation and maintenance of the state in which the vesicle membrane is bound to the planar bilayer when the membranes contain negatively charged lipids. This attachment is stronger than the interaction between zwitterionic membranes.