Abstract Optical rotatory dispersion of ferri-, oxy-, and deoxyhemoglobin was significantly reduced by commonly used concentrations of the anesthetic halothane, 2-bromo-2-chloro-1,1,1-trifluoroethane, at physiological pH in the 220- to 300-nm region. There was no significant effect of halothane on the conformation of the α chain whereas the helicity in the β chain of hemoglobin was significantly decreased in the presence of halothane at 23°. These changes were reversed by re-equilibration of the hemoglobin solution with air at 37°. The effect of the d and l enantiomers of halothane on oxyhemoglobin was the same as the racemic mixture. Poly-l-lysine in a helical configuration showed a small reduction in helicity on exposure to halothane, but poly-l-glutamic acid in both a helical and random coil configuration showed no significant change in configuration on exposure to halothane.