The three-dimensional structure of the 146-residue form of human basic fibroblast growth factor (bFGF), expressed as a recombinant protein in yeast, has been determined by x-ray crystallography to a resolution of 1.8 A. bFGF is composed entirely of beta-sheet structure, comprising a three-fold repeat of a four-stranded antiparallel beta-meander. The topology of bFGF is identical to that of interleukin 1 beta, showing that although the two proteins share only 10% sequence identity, bFGF, interleukin 1, and their homologs comprise a family of structurally related mitogenic factors. Analysis of the three-dimensional structure in light of functional studies of bFGF suggests that the receptor binding site and the positively charged heparin binding site correspond to adjacent but separate loci on the beta-barrel.