Several important reasons have contributed to the current interest in manipulating the composition of cows milk, particularly the ratio of milk protein to fat. Current market forces are still influenced with the introduction of production quotas on milk volume which occurred in the mid 80's. This led to a payment scheme based on the sale of milk solids, whilst the more recent introduction of a quota on milk fat sales has provided a major disincentive to overproduce milk fat and specifically milk with a high fat content. Most recently, the pricing structure of milk was changed, with the reward for lactose being removed and replaced by an increased price differential between protein and fat. Thus to the dairy farmer, other than those producing Channel Island milk, increasing the protein content of milk appears the only option with which to increase the ex-farm gate value of the product. At the same time, consumer choice continues to have a major impact on the demand for milk and milk products and the increasing sales of semi-skimmed milk is one consequence of the Nation's concern regarding over-consumption of fats, especially those of animal origin.