The absence from the medium of any of the 13 amino acids essential for cell growth has an inhibiting effect on the multiplication of adenovirus 9-15 and adenovirus I in HeLa cell cultures. The inhibition is accentuated by previous amino acid starvation of the cultures. Whereas with arginine deprivation, the arginine pool inside the cells is at a minimum within 30 min, the cells are assumed to adapt slowly to the new metabolic state, which is characterized by an increased 'turnover' of protein synthesis. With arginine deficiency and in Hanks' BSS some synthesis of virus and capsid proteins takes place. Quantitative and possibly qualitative differences between the influence of the various deficient media were observed. The experiments rule out DNA synthesis as a primary cause of the amino acid deficiency effect. They lead to the hypothesis that arginine deficiency inhibits the formation of an essential protein which is synthesized very late in the infectious cycle under complete MEM.