Thymosin beta 4 (T beta 4) is a major intracellular G-actin-sequestering peptide. There is increasing evidence to support important extracellular functions of T beta 4 related to angiogenesis, wound healing and cardiovascular regeneration. We investigated the expression of 'T beta 4' and 'thymosin beta 10', a closely related peptide, during skeletal muscle regeneration in mice and chemotactic responses of myoblasts to these peptides. The mRNA levels of 'T beta 4' and 'thymosin beta 10' were up-regulated in the early stage of regenerating muscle fibres and inflammatory haematopoietic cells in the injured skeletal muscles of mice. We found that both T beta 4 and its sulphoxized form significantly accelerated wound closure and increased the chemotaxis of C2C12 myoblastic cells. Furthermore, we showed that primary myoblasts and myocytes derived from muscle satellite cells of adult mice were chemoattracted to sulphoxized form of T beta 4. These data indicate that muscle injury enhances the local production of T beta 4, thereby promoting the migration of myoblasts to facilitate skeletal muscle regeneration.