Abstract: Cows are milked by automatic machinery, by diverting the milk initially flowing, to a bypass tank, for a predetermined period of time. The milk is then diverted to a main vessel, to which it flows throughout the milking cycle unless the color of the milk changes as determined colorimetrically, in which case flow of milk is switched back to the bypass tank. Change of color is indicative of the presence of blood or pus in the milk.