A heparin-binding Mr 25,000 immunoreactive bFGF-like protein (ir-bFGF) is recognized in adult rat liver extract by affinity-purified polyclonal anti-human placental bFGF antibodies. Hepatic levels of this protein increase 4-fold in regenerating rat liver during the first 48 h after partial hepatectomy. Also, they appear to be higher in embryonic than in newborn or in adult rat liver. Mr 25,000 ir-bFGF from regenerating rat liver, partially purified by heparin-affinity chromatography, induces plasminogen activator activity and cell proliferation in transformed fetal bovine aortic endothelial GM 7373 cells and competes with Mr 18,000 [125I]bFGF for the binding to high affinity bFGF receptors. The data indicate the presence in rat liver of a high molecular weight form of bFGF whose expression is modulated during embryonic development and liver regeneration.