Two terms are used to explain the stationary phase of cell growth in culture. One is contact inhibition of growth, it is proposed that contact with other cells produced a response by the plasma membrane which in turn passed a signal on to the metabolic machinery of the cell signaling it not to initiate DNa synthesis or other growth-related activities Another more neutral term is density-dependent regulation of cell growth, which is now more commonly employed to indicate the fact that as cell densities increase, cell growth potential decreases. Actually, two terms are not contradic- tory. While contact inhibition should be an important factor in the control of cell growth, cells living at higher density. i.e, packed against their neighbors, seem to require a higher concentration of growth factors in particular, concentration of serum in medium than those at lower density. Therefore occurrence of contact inhibition of growth is dependent onTwo terms are used to explain the stationary phase of cell growth in culture. One is contact inhibition of growth, it is proposed that contact with other cells produced a response by the plasma membrane which in turn passed a signal on to the metabolic machinery of the cell signaling it not to initiate DNA synthesis or other growth-related activities. Another more neutral term is density-dependent regulation of cell growth, which is now more commonly employed to indicate the fact that as cell densities increase, cell growth potential decreases. Actually, two terms are not contradic￾tory. While contact inhibition should be an important factor in the control of cell growth, cells living at higher density, i.e, packed against their neighbors, seem to require a higher concentration of growth factors in particular, concentration of serum in medium than those at lower density. Therefore, occurrence of contact inhibition of growth is dependent on