Summary In twelve trials during winter months of two successive years no measurable change in the iodine number of milk fat could be found as the result of the development of moderate or fairly pronounced oxidized flavor. It is shown by calculation that oleo-stearo lecithin in quantities usually found in sweet cream butter could not affect a measurable change in iodine number even though the double bonds of the oleic acid were completely oxidized. The evidence that the milk fat itself is not affected when oxidized flavor develops indicates that some other constituent of milk is the one in which the off-flavor arises when moderate to fairly pronounced oxidized flavor is produced.