We have examined the effect of T-ansforming Growth Factor (TGF) beta on proliferation of L6 and embryonic porcine myogenic cells. Proliferation of L6 cells was suppressed by both TGF beta-1 and TGF beta-2 in a dose-dependent manner. Half-maximal suppression of proliferation occurred at.036 ng TGF beta-1/ml and.06 ng TGF beta-2/ml. Maximal inhibition (60% suppression of proliferation for TGF beta-1 and 52% for TGF beta-2) occurred between.1 and.3 ng/ml for each growth factor. Suppression of proliferation was completely abolished in the presence of an anti-TGF beta antibody that inhibited the biological activity of TGF beta-1 and TGF beta-2. When we evaluated the effect of TGF beta-1 on proliferation of embryonic porcine myogenic cells we obtained results which were very similar to those obtained for L6 cells. Insulin-like growth factor (IFG)-I stimulated proliferation of L6 cells in a dose-dependent manner in serum-free, defined medium. However as little as.02 ng TGF beta-1/ml detectably suppressed this stimulation and.3 ng TGF beta-1/ml caused a 60% reduction in cell number in cultures treated with 30 ng IGF-1/ml. Thus TGF beta-1 significantly suppressed IGF-l-stimulated proliferation of L6 cells.