A 17kD heparin-binding protein (HBp17) has a biphasic dose-dependent effect on DNA synthesis in 3T3 cells. Maximal stimulation of DNA synthesis occurs at 8ng/ml HBp17, but a half-maximal inhibition occurs at ∼500ng/ml. This inhibition can easily be reversed by addition of 400pg/ml aFGF or 100pg/ml bFGF, whereas EGF had no effect. This biphasic action of HBp17 was also seen in human umbilical vein endothelial cells (HUVEC), whereas it was not found in the malignant cell line, A431-AJC. The functional relationship between HBp17 and FGF is discussed.