The conformation of the 153-residue form of human basic fibroblast growth factor (bFGF) was studied with circular dichroism (CD) and sequence prediction methods. The far-UV CD spectrum with a minimum at 202 nm resembled that of an unordered polypeptide/protein or a protein rich in distorted antiparallel beta-sheets. Analysis of the CD spectrum by the least-squares method of Chang et al. (1978) and the CONTIN program of Provencher and Glöckner (1981) suggested that about one half of the molecule consisted of beta-sheet and there was no alpha-helix. These estimates agreed with the prediction by the sequence method of Garnier et al. (1978) using decision constants based on CD results. bFGF had an unusual CD band at 187 nm, which disappeared upon ionization of Tyr side chains at pH 11.7. It also had another unusual property of irreversibly converting the CD spectrum to a helix-like one with a double minimum at 205 and 215 and a maximum at 189 nm upon heating the solution to above 55 degrees C. The helicity was also enhanced in trifluoroethanol and in sodium dodecyl sulfate. The mutant bFGF in which cysteines 76 and 94 were replaced by serine residues had essentially the same properties as the wild-type.