Receptor specificity is an essential mechanism governing the activity of fibroblast growth factors (FGF). To begin to understand the developmental role of FGF-9/glial activating factor, we have cloned and sequenced the murine FGF-9 cDNA and expressed the protein in mammalian cells and in Escherichia coli. We demonstrate that the FGF-9 protein is highly conserved between mouse and human. Receptor specificity was determined by direct binding to soluble and cell surface forms of FGF receptor (FGFR) splice variants and by the mitogenic activity on cells, which express unique FGF receptor splice variants. Our data demonstrate that FGF-9 efficiently activates the 'c' splice forms of FGFR2 and FGFR3, receptors expressed in potential target cells for FGF-9. Significantly, FGF-9 also binds to and activates the 'b' splice form of FGFR3, thus becoming the first FGF ligand besides FGF-1 to activate this highly specific member of the FGF receptor family.