Human CD23 (also known as Fc epsilon RII) is a 45,000 MW glycoprotein with homology to C-type animal lectins. It is involved in B-cell differentiation and IgE regulation, and is naturally cleaved to give soluble products of 37,000, 33,000, 29,000, 25,000 and 16,000 MW. Previous work has suggested that the region between the transmembrane sequence and the extracellular lectin head is capable of forming an alpha-helical coiled coil, one of the main consequences of which would be formation of dimers or trimers. Here we present protein-protein cross-linking data showing that CD23 forms trimers on the cell surface and hexamers in solution, and we use several different fragments to determine the regions of the protein involved in this self-association. The region of the putative coiled coil is indeed responsible for trimerization, with additional interactions between the lectin heads resulting in the formation of hexamers observed in solution.