Objective To investigate the effect of macrophageconditioned medium on the growth of mouse myoblasts. Methods After a 3day intraperitoneal injection with a mixture of muscle homogenate and starch, macrophages were obtained from mouse peritoneal cavity and cultured for 48 hours in serumfree DMEM/F12. Then the cultured medium was harvested. Myoblasts were obtained from newborn mice skeletal muscle and grown in DMEM/F12 supplied with 10% FBS for 3648 hours, and then exposed to 0.5% FBS medium with or without macrophageconditioned medium. Cells were harvested at 72 hours later, their growth was observed with Wright staining, LDH cytochemistry and immunocytochemistry. Results Macrophageconditioned medium can maintain the normal morphology and the activity of myoblasts grown in low concentration of serum and promote their proliferation. There was a significant difference between average absorbance and integral absorbance of LDH positive cells of the two groups(P<0.01 and P<0.000 1 respectively). The experiment group contained more positive cells compared with the control group, identified by immunocytochemistry for desmin.Conclusion Ma