Calmodulin (CaM), a multifunctional calcium binding protein with no known enzymatic activity, has been purified to homogeneity from bovine adrenal cortex. The purification included anion exchange on DE-52 cellulose, ammonium sulfate precipitation, and separation by molecular sieving on Sephadex G-150. The yield of CaM from 900 g of whole adrenal was 150 mg. Adrenocortical CaM showed a molecular weight of 18,000 on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, an isoelectric point of 4.1, and demonstrated a characteristic shift in mobility on polyacrylamide gels in the presence of calcium. The spectral properties of adrenocortical CaM differed slightly from those of CaM isolated from bovine brain. Minor differences were observed in peptide maps and amino acid composition between adrenocortical and brain CaM, but adrenocortical CaM contained a single trimethyllysine residue characteristic of all mammalian forms of CaM isolated to date. Adrenocortical CaM is biologically active in the stimulation of activator-deficient phosphodiesterase, and showed a half-maximal effective concentration (EC50) of 3 nm for stimulation of adenylate cyclase from Bordetella pertussis.