The beta-thymosins are peptides of about 5 kDa molecular mass. Thymosin beta4 (Tbeta4) is the most ubiquitous member of this family and composed of 43 residues. Initially the beta-thymosins were supposed to be specifically produced and released by the thymic gland and to possess hormonal activities modulating the immune response. However, it was later noticed that beta-thymosins are present in the cytoplasm of almost all eukaryotic cells. Especially high concentrations of Tbeta4 were detected in hematopoetic cells, like polymorpho-nuclear leucocytes and in platelets. In these cells the main intracellular function of the beta-thymosins is to bind to monomeric actin and to inhibit its polymerization to filamentous actin. Thus Tbeta4 allows resting eukaryotic cells to maintain a high concentration of monomeric actin, although the intracellular ionic conditions would favor its almost complete polymerization to F-actin. Thereby monomeric actin is sequestered from the dynamic assembly and disassembly processes of the actin cytoskeleton that constantly occur intracellularly.