Leukaemia inhibitory factor (LIF) has been reported to specifically enhance myoblast proliferation in vitro and increase the number and size of myotubes in regenerating skeletal muscle in vivo. The present study specifically tests the effect of LIF on myoblast replication in vivo. Administration of exogenous LIF by slow release alginate gels in vivo sustained the level of myoblast proliferation at 2 days in regenerating crush-injured muscle. Since the extracellular matrix (ECM) plays an important role in regulating the effects of many growth factors, the hypothesis was tested, both in vivo and in vitro, that some of the beneficial effects of LIF are mediated by modulation of the ECM. The effects of LIF in vivo on the amount and localisation of the ECM molecules, fibronectin, tenascin-C, collagen type IV and laminin were assessed by immunohistochemistry on regenerating skeletal muscle but no influence of LIF on ECM composition was observed. In tissue culture, LIF increased BALB/c myoblast proliferation at day 3 on culture dishes coated with Matrigel and also increased the viability in vitro of BALB/c myoblasts grown under suboptimal conditions. Quantitation of the ECM produced by cultures (enzyme-linked immunosorbent assay) showed that LIF affected the amount of fibronectin, tenascin-C, collagen type IV and laminin produced by fusing myoblasts. No significant affect of LIF was seen on myotube formation either in vitro or in vivo. These combined in vitro and in vivo studies show an effect of LIF on ECM production in vitro, on myoblast survival and on in vivo myoblast replication.