AbstractAcidic and basic fibroblast growth factor (aFGF and bFGF respectively) are closely related mitogens (55% homology) of the heparin binding growth factor family. Reports of the relative potency of these growth factors and the ability of heparin to potentiate the activity of bFGF are conflicting. We have examined the effect of heparin and human recombinant aFGF and bFGF on basal and thrombin challenged release of metabolites from cultured human umbilical vein endothelial cells (HUVEC). Culture supernatant was assayed for thrombospondin, prostacyclin and PAI-1 and cell lysates were analysed for t-PA. aFGF and bFGF were equipotent in regulating the release of all metabolites studied, except thrombin stimulated release of PGI2where bFGF was more potent than aFGF in the absence of heparin. Heparin potentiated the mitogenic and metabolic effects of both bFGF and aFGF. However, heparin was not essential for the expression of the biological activity of FGF.