Basic fibroblast growth factor (bFGF) is a multi-potential growth factor whose biological activities depend on its receptor's intrinsic tyrosine kinase activity and second messengers such as the mitogen activated protein kinases. Heparin sulfate proteoglycans (HSPGs) have been demonstrated to enhance or inhibit bFGF activity. The response elicited by HSPG is related to the relative concentrations and binding kinetics for bFGF of the various pools of HSPG. The type of cellular response might depend on the specific HSPG and FGF receptor expressed on the cell surface. The specific core protein of HSPG, and tissue specific differences in heparin sulfate modification result in altered bFGF regulation.