Non-transformed or normal cells-having definite life span­and primary cultures from healthy tissues, require defined quantities of proteins, growth factors and hormones even in the best media. Cells immortalized either spontaneously or by transfection with viral sequences, produce most of these factors. Still they need some of the growth factors present in the serum. On the contrary, transformed cells-having autonomous growth control and malignant properties-synthesize their own growth factors. Addition of growth factors may even be deleterious in such cases. However, even these cultures may require factors like insulin, transferrin, lipids, etc.