A number of growth factors are involved in coordinating muscle cell proliferation and differentiation, particularly after injury and in disease. Leukaemia inhibitory factor (LIF) strongly stimulates the proliferation of myoblasts in vitro and in vivo and its expression in muscle after injury suggests that LIF may have a role as a trauma factor. The mdx mouse was used to study the effects of LIF on in vivo muscle regeneration during disease. The rationale for using trophic factors such as LIF to treat neuromuscular disease includes the understanding that these molecules show some degree of selectivity for the population of cells in which they are effective. LIF was administered to muscle of the mdx mouse using osmotic pumps implanted subcutaneously in unrestrained mice. The growth factor was continuously delivered into the vastus lateralis muscle at 7 U/mu 1 for 7 days via a catheter. The results show that LIF increased the rate of muscle regeneration in mdx mice by stimulating the formation of larger myotubes. LIF treatment also increased the number of regenerating myotubes in the perfused area. This myotrophic action indicates that LIF contributes to muscle regeneration. Together with its known neurotrophic action, LIF is a potential therapeutic agent for the treatment of neuromuscular disease.