Calelectrin is a new calcium-binding protein isolated from the cholinergic nerve terminals of the electric organ of Torpedo marmorata, which is widely distributed in nervous tissues and selectively binds to membranes, self-aggregates, and promotes calcium-induced membrane aggregation as a function of calcium concentration. We now show by immunofluorescence and immune blotting procedures that this protein is also present in human blood cells. Immunofluorescence demonstrates calelectrin in all human leucocytes, including mononuclear cells, but not in platelets or in erythrocytes. The immunofluorescence indicates an exclusively cytoplasmic location of calelectrin with a diffuse distribution and no primary association with the cytoskeleton or the cell membranes. SDS-polyacrylamide gel electrophoresis with immune blotting of fractionated blood cells (thrombocytes, mononuclear cells, granulocytes and erythrocytes) reveals the presence of a single protein crossreactive with calelectrin from Torpedo marmorata in the granulocyte and mononuclear cell fractions only. Human calelectrin has a molecular weight similar to Torpedo calelectrin (approximately 34-35 kD) and also binds to membranes in a Ca(2+)-dependent manner. Our results have several implications: (1) Calelectrin is conserved during evolution between the fish Torpedo marmorata and humans; (2) its expression in neural and mesenchymal cells points to an important functional role of the protein; (3) its absence from platelets excludes the hypothesis that it is a necessary participant in exocytosis per se and suggests some other function in Ca(2+)-triggered processes.