An analysis of subacute sclerosing panencephalitis (SSPE) lymphocytes was performed in order to find out whether T or K cells were involved in killing of 51Cr-labeled allogeneic measles virus-infected target cells. Lymphocyte donors were three patients with SSPE, 10 measles seropositive controls and 2 children with measles rash. It was found that about 75% of measles-specific cytotoxicity was lost after removal of Fc-receptor-bearing cells by adsorption onto immune complex monolayers. K cell activity (as measured by lysis of 51Cr-labeled-sensitized chicken red blood cells) was reduced to the same extent. After adsorption, the enhancing effect by specific antibody was no longer observed. Unfractionated peripheral lymphoid cells that had been treated with pronase and kept in culture 24 hr were inactive in the cytotoxicity test when compared to freshly isolated cells. However, cytotoxicity could be restored almost completely by the addition of measles antibodies. The results indicate that measles-specific cytotoxicity by peripheral lymphoid cells from all three groups of donors is mediated by K cells. It is probable that specificity is provided by a small amount of contaminating serum antibody or immune complexes.