Abstract: Human cell lines which proliferate in the absence of serum or of any macromolecular growth factors are produced by a three step procedure comprising removing serum containing culture medium from the culture of a human cell line which requires serum or macromolecular growth factors for growth, and replacing it with culture medium which is deprived of serum and of any macromolecular growth factors, feeding the adherent or non-adherent cells with serum and growth factor free culture medium using conventional cell culture conditions, culturing the cells until they propagate reproducibly and indefinitely in the absence of serum and of any macromolecular growth factors and a serum and growth factor independent cell line has been established. The novel cell lines can be used for the production of proteins, especially plasminogen activators.