While the prototype members of the fibroblast growth factor (FGF) family, FGF-1 and FGF-2 are structurally related, the structural differences between these polypeptides predict that they will ultimately exhibit different biological roles. Indeed, a significant difference between these proteins is the dependence of FGF-1 on heparin for the generation of maximal mitogenic activity. In order to gain structural insight into the issue of FGF-1 heparin-dependence, a synthetic gene encoding FGF-2 was constructed with oligonucleotides in a four-cassette format similar to a synthetic gene previously constructed for FGF-1 (Forough et al. 1992, Biochem. Biophys. Acta 1090 293-298). This strategy permitted the molecular shuffling of corresponding cassette(s) between FGF-1 and FGF-2 to yield FGF-1:FGF-2 chimeras. Three amino acid changes (Lys86-->Glu, Tyr120-->His, and Thr121-->Ala) were introduced into the synthetic FGF-2 gene by the cassette format to generate convenient FGF-1 restriction sites, but these alterations did not significantly affect the mitogenic activity or the heparin-binding affinity of the recombinant FGF-2 protein when compared with native FGF-2. Among the various FGF-1:FGF-2 chimeric constructs, one designated FGF-C(1(1/2)1 1), which represents FGF-1 containing FGF-2 amino acid residues 65 to 81, displayed FGF-1-like heparin-binding affinity but it did not require the addition of exogenous heparin to manifest its mitogenic activity. These data suggest that the sequence within residues 65 and 81 from FGF-2 significantly contributes to the heparin-dependent character of FGF-1.