The invention relates to a process for the culturing of cells, preferably E1-immortalized HER cells, more preferably PER.C6 cells in a reactor in suspension in a cell culture medium, wherein the cells produce a biological substance, preferably an antibody, wherein at least one cell culture medium component is fed to the cell culture and wherein the cell culture comprising the cells, the biological substance and cell culture medium is circulated over a separation system and wherein the separation system separates the biological substance from substances having a lower molecular weight than the biological substance and wherein the biological substance is retained in or fed back into the reactor. Preferably part of the substances of lower molecular weight is continuously removed from the cell culture.