Abstract: Cell cultures are described which proliferate in a serum- and protein-free environment and which are characterized by the circumstance that the concentration of at least one cell-cycle-regulatory protein is increased during at least one period of the cell-cycle. Methods for the production of such cell cultures are presented. With these methods cell lines can be transferred to serum- and protein-free growth that are already in use in a production process which, prior to the use of these methods, requires serum or proteins added to the basal medium.