An adenylate kinase (AK) activity modulated by calcium ion concentration has been found associated to the disk membranes of the rod outer segment of bovine retina. A maximum activity of about 80 nmol ATP produced per min per mg protein was found at physiological calcium concentrations. Preliminary experiments suggest that the membrane binding is presumably promoted by fatty acylation of the protein. In fact, a protein with a molecular weight corresponding to the disk adenylate kinase was recognized by a polyclonal antiserum against the first 15 N-terminal amino acids of AK1β, a membrane-associated isoform of adenylate kinase, which belongs to the N-terminus myristoylated protein family. The adenylate kinase activity was also measured directly on the protein band transferred to nitrocellulose by Western blot.