The interaction of G-actin or F-actin with H-meromyosin was studied at a low concentration of KC1 (10 mM) by flow birefringence, viscosity, sedimentation and electron microscopy. The polymerization of G-actin was greatly accelerated by H-meromyosin in 10 mM of KCI, and the amount of F-actin thus polymerized was approximately proportional to the amount of H-meromyosin added up to a weight ratio of 1: 4. The actin-H-meromyosin complex was found not to depolymerize even at a very low protein concentration at low ionic strength. The number-average particle length of the actin-H-meromyosin complex was about 0.8μ and the weight-average length was about 1.6μ, when G-actin was added to H-meromyosin. The actin-H-meromyosin complex at low ionic strength showed two schlieren peaks with about 80S and 150—200S in the sedimentation pattern. Upon addition of ATP, the peak of F-actin with 40S was observed.