A calcium-sensitive actomyosin was prepared from bovine aortic muscularis. The results of applying filament displacement tests indicate that bovine arterial actomyosin possesses predominantly a calcium regulatory system associated with the myosin filament, but also may possess a calcium regulatory system associated with the actin filament. Calcium binding to highly purified myosin preparations could not account for the calcium binding to calcium-sensitive actomyosin. A calcium-insensitive actomyosin preparation, prepared by omitting dithiothreitol from the isolation procedure, showed a definite loss of tropomyosin but no change in calcium binding relative to the calcium-sensitive actomyosin.