Calelectrin is a protein that can be purified to homogeneity from the cholinergically innervated electric organ of Torpedo marmorata where it is present in large amounts. It has been shown to bind to the membranes of the electric organ in a Ca2+-dependent and specific manner. Using the purified protein we now report that it is specifically self-aggregated by Ca2+ in micromolar concentrations but not by Mg2+ at much higher concentrations. Sr2+ is also completely inactive, while Ba2+ and the trivalent lanthanides Tb3+, Eu3 +, and La3+ can substitute for Ca2+. Calelectrin also greatly enhances the Ca2+-induced aggregation of isolated synaptic vesicle membranes from the cholinergic nerve terminals of T. marmorata and of chromaffin granule membranes from the bovine adrenal medulla. The potentiation of membrane aggregation is mainly due to the appearance of a fast aggregatory phase in the presence of calelectrin . It is saturable with respect to calelectrin and can be demonstrated at very low calelectrin concentrations, suggesting a specific calelectrin membrane-binding component. This component seems to be of lipid nature since the aggregation of total extracted lipids from Torpedo electric organ and from chromaffin granules could also be enhanced by calelectrin . The Ca2+-induced self-association of calelectrin and its aggregation enhancing effect may be of great importance to the structural organization of neural and secretory cells and the mechanism of exocytosis.