A multistage countercurrent diafiltration process for whey protein concentrates (WPC) production is presented. Experimental tests of whey diafiltration have validated a mathematical model for the concentration of components in the permeate for the case of washing with a dilute solution containing whey components. This model has been used together with material balances in order to simulate the multistage process. Water saving with respect to a single-stage conventional diafiltration was demonstrated. In the case of a WPC production of 95.5% from the whey of a local dairy farm which contained 7.7 g/L proteins, 43 g/L lactose, 1.1 g/L nonproteic nitrogen, and 5.4 g/L ashes, the necessary volumes of water, VD (volume of water per volume of treated solution), needed decreased from 5.4 for a single stage to 2.9 for a two-stage process down to 1.5 for a six-stage process. The adopted procedure can be easily applied to any diafiltration process.