The role of tumor necrosis factor-alpha (TNF-α), an important mediator of the inflammatory response after injury, was investigated in regenerating skeletal muscle. The pattern of expression of TNF-α during muscle regeneration was examined by immunohistochemistry in tissue sections of crush-injured or transplanted muscle autografts and in primary cultures of adult skeletal muscle. TNF-α was highly expressed in injured myofibers, inflammatory cells, endothelial cells, fibroblasts, and mast cells. Myoblasts and myotubes also expressed TNF-α in primary muscle cultures and tissue sections. The essential role of TNF-α and its homologue lymphotoxin-alpha (LT-α) during muscle regeneration was assessed by basic histology in TNF-α(–) and TNF-α(-/-)/LT-α(-/-) mice. No difference was apparent in the onset or pattern of muscle regeneration (i.e., inflammatory response, activation and fusion of myoblasts) between the two strains of null mice or between nulls and normal control mice. However, both strains of null mice appeared more prone to bystander damage of host muscle and regeneration distant from the site of injury/transplantation. Although expression of TNF-α may play an important role in muscle regeneration, the studies in the null mice show that redundancy within the cytokine system (or some other response) can effectively compensate for the absence of TNF-α in vivo. (J Histochem Cytochem 49:989–1001, 2001)