1. Oxidation-reduction potential methods have been applied to a study of the blue-purple pigment present in solution in the blood and in the tissue cells of the nudibranch Chromodoris zebra.
2. The blue-purple pigment and its yellow reduction product form a reversible system whose Eo' = x0.102 volts at pH 7.0 and whose valence change from oxidant to reductant appears to be one.
3. The system is unlike oxyhemoglobin-hemoglobin in the mode of oxygen transfer. Its rôle as a possible respiratory material is discussed.