The secondary structure of Cerebratulus lacteus toxin B-IV, a neurotoxic polypeptide containing 55 amino acid residues and four disulfide bonds, was experimentally estimated by computer analyses of toxin circular dichroism (CD) and laser Raman spectra. The CD spectrum of the toxin displayed typical alpha-helical peaks at 191, 208, and 222 nm. At neutral pH, the alpha-helix estimates from CD varied between 49 and 55%, when nonrepresentative spectrum analytical methods were used. Analysis of the laser Raman spectrum obtained at a much higher toxin concentration yielded a 78% alpha-helix estimate. Both CD and Raman spectroscopic methods failed to detect any beta-sheet structure. The spectroscopic analyses revealed significantly more alpha-helix and less beta-sheet for toxin B-IV than was predicted from its sequence. To account for the difference between the 49-55% helix estimate from CD spectra and the 78% helix estimate from the Raman spectrum, we postulate that some terminal residues are unfolded at the low toxin concentrations used for CD measurements but form helix at the high toxin concentration used for Raman measurements. Our CD observations showing that Cerebatulus toxin B-IV helix content increases about 15% in trifluoroethanol or at high pH are consistent with this interpretation.