A number of suppositions concerning the cause of the discoloration of fish-oils have been published by previous investigators. According to the results of our experimental study here reported, a remarkable discoloration of fish-oils takes place when we make the autoxydizing fish-oils react with trimethylamine which is one of the volatile bases. On the other hand, the autoxdizing fish-oils are found to produce aldehydes. From these facts is induced our supposition, that the red discoloration of fish-oils is brought about by the chemical combination of aldehydes and volatile bases.