Gelsolin and thymosin β4 appear to be two important actin‐associated proteins involved in the regulation of actin polymerization. It has been widely demonstrated that thymosin is the major cellular actin‐sequestering factor shifting the polymerization equilibriuni of actin towards a monomeric state. At the same time gelsolin, a Ca2+and inositol phosphate sensitive protein, regulates actin filament length. The interactions of these two proteins with actin are rather complex and require the participation of several complementary peptide sequences. We have identified a common motif, (I. V)EKFD, in the two proteins in the functional sequences so far examined. Gelsolin‐ and thymosin β4‐related peptides including the common motif were synthesized and their structural and functional properties studied. These two sequences exert a major inhibitory effect on salt‐induced actin polymerization. We used circular dichroism and Fourier‐transform infrared spectroscopy to show that the two synthetic peptides present some secondary structure in solution. As far as the peptide derived from the thymosin sequence was concerned, α‐helical structure was induced by trifluoroethanol as observed with the full‐length molecule. These experiments underscore the importance of the conformational state of peptide fragments in their biological activities. ELISA and fluorescence measurements have been used to identify the binding regions of these fragments to aC‐terminal region (subdomain 1) of the actin sequence. Our results also emphasize the relationship between the propensity of small sequences to form secondary structures and their propensity for biological activity as related to actin interaction and inhibition of actin polymerizati