Soy-based formulas are consumed by growing numbers of infants and used as regular food supplements in livestock production. Moreover, constituent dietary phytoestrogens may compete with endogenous estrogens and affect individual growth. This study aimed to investigate the in vitro effects of isoflavones in comparison with estrogens on the proliferation of porcine satellite cells derived from neonatal muscle. After 7 h of exposure in serum-free medium, 17beta-estradiol (1 nM, 1 microM), estrone (1 microM), and daidzein (1, 100 microM) slightly decreased whereas 100 microM genistein substantially lowered DNA synthesis. Declines in DNA amount were observed with genistein (1, 100 microM) and daidzein (100 microM). After 26 h of exposure, 100 microM genistein reduced DNA synthesis, whereas it was increased by 10 microM genistein and 10 and 100 microM daidzein. In the case of 10 microM genistein and 100 microM daidzein, these increases apparently resulted from the repair of damaged DNA. Genistein and daidzein (100 microM) reduced protein synthesis, caused a G2/M phase block, and decreased DNA amount in association with higher rates of cell death partially resulting from apoptosis. Conclusively, isoflavones at concentrations of greater than 1 muM act as inhibitors of porcine skeletal muscle cell proliferation.