Objective：To study systematically the role of skeletal muscle cells in synthesizing, processing and secreting recombinant proteins in vitro. Methods: Primary murine myoblasts from SCID mice (SCID-MB) and C2C12, a mouse myoblast cell line, were transfected with muscle specific (pdLMe4bAhⅨm) or non-specific (LⅨSN) vectors encoding human factor Ⅸ (FⅨ). The capacities of the transgenic cells in producing recombinant FⅨ were compared in the levels of intracellular and secreted FⅨ protein as well as FⅨ mRNA in primary muscle cells and C2C12 cells. Results: Both primary muscle cells and C2C12 cells can efficiently translate and process FⅨ protein. Myotubes derived from the SCID-MB produced-2000 ng FⅨ．(106 cells)-1．(24 h)-1 in culture with specific activity of 95%？103%. C2C12 cells can synthesize and process recombinant FⅨ as efficiently as primary muscle cells, but their secretion ability is less efficient in comparison with primary cells. Conclusion: The primary cells should be used in gene therapy related research projects and care should be taken while explaining the results derived from established cell lines.