Fluorescing substances developed in the following materials during storage: high protein foods, represented by dried whole milk powder, dehydrated pork, and soya flour; high carbohydrate foods, represented by dried banana flakes and dried parsnips; and a mixed foodstuff, represented by ration biscuits.The only change occurring in stored shortenings was a decrease of fluorescing substances in hydrogenated linseed oils. Serum extracted from rancid butter had a higher fluorescence value than serum from fresh butter. In substances containing a high proportion of fat, fluorescence values bore little relation to deterioration as assessed by peroxide oxygen determinations.Fluorescence tests were unsatisfactory for dried milk powders and soya flour. However, they may prove useful as a measure of quality for dehydrated pork, dried banana, dried parsnips, ration biscuits, and butter. Fluorescence measurements may also detect reversion in hydrogenated linseed oil shortenings.