establishing a cell culture by inoculating a bioreactor with mammalian cells expressing a recombinant protein,

maintaining the cell culture by perfusing fresh cell culture medium into the bioreactor, passing the cell culture through a filter and collecting a permeate,

wherein a null permeate is initially collected until a first predetermined parameter selected from time, viable cell density, packed cell volume or titer is reached, at which time a harvest permeate comprising the recombinant protein is collected for a predetermined time,

this is followed by alternately collecting a null permeate until a second predetermined parameter selected from time, viable cell density, packed cell volume or titer is reached, then collecting a harvest permeate for a predetermined time,

wherein the alternating collection of null permeate and harvest permeate continues until the cell culture is terminated,

wherein when the null permeate is collected, the filter is a hollow fiber filter having a pore size or molecular weight cut off that retains the recombinant protein in the bioreactor, and when the harvest permeate is collected, the filter is a hollow fiber filter having a pore size or molecular weight cut off that does not retain the recombinant protein in the bioreactor.