Incisional wounds in rats treated locally with thymosin beta4 (Tbeta4) healed with minimal scaring and without loss in wound breaking strength. Treated wounds were significantly narrower in width. Polarized light microscopy treated wounds had superior organized collagen fibers, displaying a red birefringence, which is consistent with mature connective tissue. Control incisions had randomly organized collagen fibers, displaying green birefringence that is consistent with immature connective tissue. Immunohistology treated wounds had few myofibroblasts and fibroblasts with alpha smooth muscle actin (SMA) stained stress fibers. Polyvinyl alcohol sponge implants placed in subcutaneous pockets received either carrier or 100 microg of Tbeta4 on days 2, 3, and 4. On day 14, treated implants revealed longer, thicker collagen fiber bundles with intense yellow-red birefringence by polarized light microscopy. In controls, fine, thin collagen fiber bundles were arranged in random arrays with predominantly green birefringence. Controls contained mostly myofibroblasts, while few myofibroblasts appeared in Tbeta4 treated implants. Electron microscopy confirmed both cell types and the degree of collagen fiber bundle organization. Our results demonstrate that Tbeta4 treated wounds appear to mature earlier and heal with minimal scaring.