THE concept of the continuous renewal of the plasma membrane of a cell has been well supported by studies on the rate at which histocompatibility antigens on the surface of the cell are replaced1–4. Thus, if HL-A or H-2 antigens are removed by treatment with proteolytic enzymes, about 6 h elapse before the cell resynthesises them and they are again detectable with the appropriate antisera. When lymphocytes are incubated with a labelled anti-HL-A serum, label is gradually released during the next 6–8 h and the complement-dependent cytotoxic effects of an anti-HL-A serum are lost in a similar way; we call this ‘escape from sensitisation’. This suggests that the turnover of the membrane of a cell may be primarily responsible for the ability of the cell to deal with antibodies binding to it, but the mechanisms involved are by no means clear. It has been suggested that antibody could either be released from the surface of the cell as an antigen-antibody complex or that it might be internalised, catabolised and then released, but neither of these ideas has been fully investigated. The early fate of fluorescent material has however been extensively studied5,6,7.