Satellite cells were isolated from 20-d embryonic chick breast muscle via a Percoll density gradient fractionation technique. Culturing of these cells gave rise to at least 89% fusion (myotube nuclei number/total nuclei number). Proliferation of cultured satellite cells (indicated by myotube nuclei number) was increased in a dose-dependent manner when fibroblast growth factor (FGF) was included in the medium (25 to 200 ng/ml). Similar cultures were used to examine the effects of ractopamine and isoproterenol on satellite cell proliferation. Ractopamine and isoproterenol were added to culture medium (10(-11) to 10(-4) M) 24 h after initial plating. After a 72-h treatment period, the treatments were removed and replaced with a medium to promote fusion for 48 h. Cells then were fixed and stained, and myotube and total nuclei were counted. In later experiments, ractopamine and isoproterenol each increased (P less than .01) myotube nuclei number vs that observed in control cultures by 2.3 and 2.1 times, respectively. Similar differences were observed with total nuclei number. The number of myotube nuclei observed in cultures treated with 10(-6) M ractopamine or isoproterenol was reduced (P less than .01) by 25.4 and 23.6%, respectively, when propranolol, a beta adrenergic antagonist, was included at 10(-5) M with the respective agonist. These results indicate that ractopamine and isoproterenol each enhance the proliferative activity of chick satellite cells in culture and that the beta adrenergic receptor mediates this proliferative effect.