It was demonstrated that the pattern of circular dichroism (CD) of poly-D-glutamic acid with peaks at 222, 206 and 190 mμ represented a mirror image of that of poly-L-glutamic acid. This result suggested that the three CD peaks observed for α-helical poly-glutamic acids reflect the statues of their backbone conformation. Based on this finding, attempt was made to estimate helical contents of several protein by measuring the CD amplitude at 222 mμ. The results obtained were compared with those obtained by the optical rotatory dispersion. The former results were in good agreement with the latter. The CD peak observed at 280 mμ for several proteins was attributed to tyrosyl residues. By comparing with the CD pattern of poly-L-tyrosine it was concluded that the band at 280 mμ of proteins is due to buried tyrosyl residues. Likewise, the CD band at around 290 mμ was attributed to buried tryptophanyl residues. The thermal denaturation of several proteins was studied by following the CD bands at 280 and 290 mμ