Abstract: High quality protein products are obtained from whey by subjecting whey to cross flow filtration in microfilter. The whey and retentate are brought to circulate in a circulation path on one side of the membrane area of the microfilter, while whey which has passed through the membrane area (permeate) is circulating in a second path on the other side of the membrane area in such a way that the pressure drop through the whole membrane area is kept constant and below 0.8 bar. With this technique, there is obtained a fractionation of milk serum protein such that denatured milk serum protein and fat are retained in the retentate and undenatured milk serum protein passes through the membrane into the permeate. With a subsequent treatment there is obtained a whey protein product which is rich is .alpha.-lactalbumin and .beta.-lactoglobulin and has a low fat content.