Thymosin beta 4 containing 43 amino-acid residues belongs to a family of highly homologous peptides present at high concentrations in various species, cells, and tissues. Safer et al. [J. Biol. Chem. 266, 4029-4032 (1991)] have shown that thymosin beta 4 is an actin-sequestering peptide. Because DNase I is inhibited by G-actin and not by F-actin we employed this enzymatic assay to determine the actin sequestering properties of 4 other thymosin beta 4-like peptides and fragments of thymosin beta 4 generated by enzymatic digestions. Thymosin beta 4 sequesters G-actin at a 1 to 1 ratio an thereby inhibits its polymerisation to F-actin in high salt solution. The oxidation of the single methionine residue at position 6 does not abolish its actin-sequestering properties. However neither thymosin beta 4 24-43 nor thymosin beta 4 13-43 inhibit the polymerisation of G-actin. We conclude from this that some structural features in the amino-acid sequence of thymosin beta 4 before position 13 are obligatory for its biological function. Oxidized thymosin beta 4 (beta 4-sulfoxide) as well as four other thymosin beta 4-like peptides were shown to be actin-sequestering peptides like thymosin beta 4.