Recently, Heizmann et al. (Proc. Natl. Acad. Sci. U. S. 78, 74-77 (1981)) reported that muscle β-actinin and serum albumin of the chicken are indistinguishable by physicochemical and immunological criteria. It should, however, be stated that the protein the above authors called β-actinin was entirely different from genuine muscle β-actinin (Maruyama et al. (1977) J. Biochem. 81, 215-232). In the present study, it was experimentally shown that chicken serum albumin does not have any of the actions of β-actinin: inhibition of reassociation of F-actin fragments, retar-dation of depolymerization of F-actin, instability of F-actin, acceleration of polymerization of G-actin, and formation of Mg polymer. The role of muscle β-actinin, a heterodimer of 37, 000 and 34, 000 daltons, in the regulation of myofibrillar structure is summarized.