We examined the combined effects of IGF-I and EGF on hyaluronic acid synthesis using cultured pericardial mesothelial cells from rabbit pericardial cavity. Combined exposure of pericardial cells to IGF-I and EGF cooperatively increased hyaluronic acid synthesis and the level of hyaluronic acid synthase activity over that seen with or without IGF-I or EGF alone, but did not significantly stimulate the sulfated glycosaminoglycan synthesis. Increase in the level of hyaluronic acid synthesis and hyaluronic acid synthase activity induced by these growth factors was blocked after pretreatment with a tyrosine-specific protein kinase inhibitor, genistein. Genistein had no direct inhibitory effect on hyaluronic acid synthase activity. These results suggest that cooperative enhancement of hyaluronic acid synthesis induced by combined use of IGF-I and EGF could be mediated by a receptor tyrosine kinase-involved transmembrane signaling process that is responsive to IGF-I and EGF.