Three peptides containing the putative Ca 2 + binding loops, I, II and III, respectively, of a photoprotein, aequorin, from jellyfish Aequorea victoria were synthesized by a solid-phase procedure. The peptides bound Ca 2 + with dissociation constants or 10 - 3 to 10 - 4 M, providing evidence for the assumption that Ca 2 + binding loops are actually responsible for the binding of Ca 2 + . When the highly conserved 6th glycine residue in the 12-residue loops was replaced by arginine, no large effect was observed on Ca 2 + binding. Exposure to a hydrophobic environment and the binding or Ca 2 + brought about conformational changes to the peptides.