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 lymphocytes^2,3. 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 In a cytotoxicity assay devised to study cellular immunity to measles in vitro (unpublished), we observed that lymphocytes seemed to cluster around measles-infected fibroblasts in far greater numbers than would be expected if only specifically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.