A study was made to determine the source of milk contamination when dairy barns were sprayed with 2.5% of DDT. Three expts. showed that the insecticide was actually secreted in the milk and did not get into it after milking due to mishandling of the milk or milking equipment; that no contamination resulted from inhalation of the insecticide by the cows; and that the spray residues left on the feed trough resulted in the secretion of the insecticide in the milk.