1. The red carotenoprotein present in the native exoskeleton of the crayfish (Cambarus clarkii) seemed to be a derivative resulted from the oxidation of blue or purple carotenoproteins.
2. The red carotenoprotein extracted from the exoskeleton was different in physical properties from the reddened one resulted by boiling.
3. The red carotenoproteins resulted from boiling and autoxidation had a lesser carotenoid content and showed a higher molecular weight than the bluish carotenoproteins.
4. It was found that the carotenoprotein reddened by heating at 100°C was partially reconverted to the original bluish carotenoproteins.
5. The presence of 0.005 M Fe++, Fe+++, Sn++, Hg++, and Cu++ caused irreversible denaturation of the carotenoproteins.