We examined the physico-chemical properties and the functions of beta-actinin by using a beta-actinin preparation having the same properties as those reported by Maruyama et al. (J. Biochem. 81, 215-232, 1977). beta-Actinin was composed of two components with molecular weights (estimated by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis) of 35,000 and 31,000 daltons. Their isoelectric points in 8 M urea were, respectively, 5.9 and 5.4, clearly distinguishable from those of tropomyosin, troponin T and some enzymes having similar molecular weights. beta-Actinin suppressed the polymerization of actin onto the free end, i.e., the pointed end, of thin filaments in an I-Z-I brush prepared by dissolving thick filaments of a myofibril at high ionic strength. Further, beta-actinin suppressed the association of actin to the whole region of an I-Z-I brush. The present study indicates that beta-actinin is composed of two components and functions as a suppressor of elongation at the pointed end of thin filaments, supporting the conclusions of Maruyama et al. (J. Biochem. 81, 215-232, 1977).