Abstract: Disclosed is a method of testing a substance for the ability to affect the formation or oligomerization of a complex comprising members of a specific binding pair, a first member of the binding pair being present on the surface of a lipid enveloped particle comprising a transferrable label, said first member of the binding pair being capable of binding to a second member of the specific binding pair present on the surface of a cell, the lipid envelope of the particle being capable of fusing with the membrane of the cell so as to transfer the label to the cell, said transfer being inhibited by formation or oligomerization of a complex between the first and second members of the binding pair, wherein the method comprises reacting the particle and the cell, in the presence of the substance under test, (under conditions which would allow for binding of the first and second members of the specific binding pair in the absence of the substance under test), and detecting the label transferred, if any; together with a