Isoflavones have been shown to inhibit protein tyrosine kinases that catalyze protein phosphorylation. The type 1 IGF receptor (IGF-1-R) and the EGF receptor (EGF-R) act as protein tyrosine kinases. This study aimed to investigate whether IGF-I and EGF stimulate the in vitro porcine muscle cell growth and whether these responses are influenced by isoflavones. Both IGF-1-R and EGF-R were found to be expressed in satellite cells from semimembranosus muscle of newborn piglets. The long-term effects (26 h) of 0.1, 1, 10 and 100 nM IGF-I or EGF, and of 10 nM IGF-I or EGF combined with 1, 10 and 100 mM genistein or daidzein on DNA synthesis were measured as 6 h-[3H]thymidine incorporation during exponential growth in serum-free medium. At 10 nM both IGF-I and EGF caused a 2- or 3.5-fold increase in DNA-synthesis (p<0.05), while combined IGF-I/EGF (10 nM) treatment revealed additive effects (4.5-fold increase). In most cases, genistein reduced the stimulating effects of IGF-I and EGF in a dose-dependent manner (p<0.05). In contrast, daidzein did not change or even slightly increased (at 100 mM) the GF responses in DNA synthesis (p<0.05). Conclusively, both IGF-I and EGF are efficient promoters of porcine muscle cell growth. Genistein, but not daidzein, depresses the GF-mediated growth stimulation, which may result from tyrosine kinase inhibition.