Myosin-binding protein C (C-protein) is a sarcomeric protein regularly associated with thick filaments of vertebrate skeletal and heart muscle and is involved in the regulating of muscle contraction. C-protein belongs to the immunoglobulin- fibronectin superfamily of proteins and it consists of 11 tandemly arranged immunoglobulin-like domains. Four Ig-domains at the N-terminus of the cardiac isoform (C0-C1-m-C2 or C0C2) influence the actin -myosin S1 interaction whereas the C-terminal domains (C7-C10) play a structural role in the sarcomere, binding to myosin thick filaments. It has been shown that C0C2 fragment stabilizes F-actin, decorating the filaments in a highly regular arrangement.We used electron microscopy and single particle image analysis to reconstruct complexes of F-actin with cardiac C-protein fragments containing 4 Ig-domains (C0C2) and 2 Ig-domains (C0C1). Both fragments regularly decorated actin filaments and image analysis has revealed substantial rod-shaped density connecting adjacent monomers along the actin filament. The fragments induced dendritic growth of actin filaments in the first several minutes of co-polymerization followed by formation of regular sheets of filaments and ordered bundles. The results will be discussed in the terms of suggested involvement of C-protein in the modulation of contractile cycles of heart muscles.