A “cardboard” taste and “linseed-oil” odor develop in whole milk which has been exposed to diffuse daylight for eight or more hours at about freezing temperature. The light apparently acts as a catalyst in the oxidation of the milk fat. The defect develops more rapidly in pasteurized than in raw milk. The presence of neither enzymes nor bacteria is necessary for the reaction. The defect develops no more rapidly in milk from cows fed heavy rations of cottonseed meal or linseed cake than in milk from cows receiving no oil feed.