Iodination of surface membrane protein has allowed the dynamics of lymphocyte surface membrane protein to be examined. A comparison has been made between the rate of membrane turnover in fresh peripheral blood lymphocytes and lymphocytes in other states. When receptor redistribution is inhibited with high concentrations of mitogens, 'rigidification' of the membrane occurs and protein turnover is very much reduced. Loss of surface membrane protein is also much slower in lymphocytes which have been stimulated with mitogens and are undergoing active DNA synthesis. It is thought that these observations may relate to immunological inactivity and may have implications for pathological unresponsiveness.