ONE of the factors which may determine whether or not a cell is susceptible to a particular virus is the presence in the cell membrane of a receptor or a binding site for the virus1. It has been demonstrated that certain viruses, including herpes and myxoviruses, can infect and multiply within lymphocytes2,3. Following von Pirquet's original observation4 it is now well recognised that acute measles infection is associated with depression of cellular (T lymphocyte-dependent) immunity. More recently, striking cytopathic changes have been described in the thymus of children during and shortly after measles5. Furthermore, blood lymphocytes from measles patients form giant cells when cultured in vitro6. In a cytotoxicity assay devised to study cellular immunity to measles in vitro (unpublished), we observed that lymphocytes seemed to cluster around measles-infected fibroblasts in far greater numbers than would be expected if only specifically measles-reactive lymphocytes were involved. We therefore decided to investigate further the nature of this clustering phenomenon. It was found that the majority of human T lymphocytes have surface binding sites for measles virus and, on binding, were cytotoxic to measles-infected cells regardless of the donor's previous experience of measles infection.