Protected fats are very useful for increasing the energy concentration of diets fed to high-yielding dairy cows without adverse effects on rumen fermentation. However, in previous studies, it has been found that although high-fat diets usually increase yields of milk and milk constituents, they can lead to a depression in milk protein content. This can be partially overcome by increasing the supply of fermentable carbohydrates, using lactose, thereby increasing microbial protein yield (Garnsworthy, 1996), or by increasing undegradable protein supply (Garnsworthy, unpublished). The objective of this experiment was to study the response of cows in mid lactation to a product containing protected fat and protein (MegaPro, Volac International) fed with or without lactose.