We identified a gelsolin-like 84 K M_r protein in the conventional actin preparation from cedures utilizing DNase I affinity column chromatography andhigh performance liquid vertebrate smooth muscle, and purified this protein from bovine aorta by simple　procedures utilizing DNase I affinity column chromatography and high performance liquid chromatography. The 84 K M_r protein reduced the high-shear viscosity of actin filaments, and shortened their lengths in a Ca²⁺-dependent manner, in relatively higher molar ratio to actin (1:100). This effect was almost unprevented by the presence of smooth muscle tropomyosin. In the lower molar ratio to actin (1:10,000), the protein reduced the viscosity of actin filaments at low-shear rates without shortening filaments in the presence of Ca²⁺. This effect was also unprotected by smooth muscle tropomyosin. Synthetic smooth muscle thin filaments consisting of gizzard actin, tropomyosin, and aorta 84 K M_r protein disassembled to shorter fragments in the presence of Ca²⁺ and reversibly assembled into long filaments when Ca²⁺ was removed by EGTA. Since conventional smooth muscle actin preparations contain the 84 K M_r protein, we discussed the necessity of a careful interpretation on the results of actin-myosin interaction with tropomyosin in vitro. Further, the possibility that the 84 K M_r protein plays an important role in some processes during physiological smooth muscle contraction, particularly long-lasting tonic contraction, was discussed.