Resonance Raman spectra are reported for suspensions of the membrane-bound protein "bacteriorhodopsin" isolated from Halobacterium cutirubrum. Most of the observed vibrations arise from the conjugated Schiff base of retinal in the chromophore. Excitation profiles for two C=C stretching vibrations at 1529 cm−1 and 1568 cm−1 show that the former is coupled to the 560 nm pigment absorption band, whereas the latter is coupled to a shorter wavelength transition. The 1529 cm−1 mode is shifted by ~40 cm−1 from retinylidenelysine in solution showing that the π electron system of the Schiff base is considerably perturbed by the protein. It is this interaction that is responsible for the bathochromic shift in the pigment.The addition of diethyl ether shifts λmax to 460–490 nm, and causes a decrease in the intensity at 1529 cm−1, with a concomitant increase at 1568 cm−1. The effect is reversible. Ether appears to weaken considerably the chromophore–protein interaction, making the chromophore more vulnerable to chemical reagents.