Porcine aortic myosin is a smooth muscle contractile protein similar to its striated muscle counterpart. Electrophoresis in sodium dodecyl sulfate indicated that the molecule consists of 3 subunits with polypeptide chain MW of 192,000, 19,000 and 15,000. At 277 nm, the absorption spectrum gave an extinction coefficient for aortic myosin of 0.558 cm2/mg; the circular dichroism spectrum of the protein indicated that aortic myosin contains .apprx. 70% of its residues in the .alpha.-helical configuration. Amino acid analysis showed that the smooth muscle myosin has significantly more arginine and leucine and significantly less valine and isoleucine than rabbit skeletal muscle myosin. Other studies yielded these data: .hivin.vapp [apparent partial specific volume] = 0.716 ml/g, [.eta.] [viscosity] = 0.213 ml/mg, s20,w [sedimentation coefficient] = 5.84 .times. 10-13 s. Similar studies with rabbit skeletal muscle myosin indicate that .hivin.vapp = 0.711 ml/g and s20,w = 6.36 .times. 10-13 s. Aortic myosin, like skeletal muscle myosin, behaves apparently hydrodynamically like a rigid rod.