A chromium release assay was used to study lymphocyte-mediated cytotoxicity against mumps virus-infected target cells in vitro. Purified lmyphocytes from randomly selected donors killed significantly more virus-infected Vero cells than non-infected cells. Lymphocyte-target cell ratios of 50 to 100:1 and incubation period from 16 to 20 hr were optimal for determination of cytotoxicity. The lymphocyte induced chromium release was not obviously correlated with serum mumps hemagglutination-inhibition titers of the effector cell donors. However, the lymphocyte reaction against virus-infected target cells seems to have an immunologic basis. Thus, the more pronounced susceptibility of mumps-infected target cells as compared to non-infected cells was not due to a cytocidal effect of the virus, since spontaneous isotope release from both target cells was the same. Also, cord blood lymphocytes which had exhibited good cytotoxicity when induced either with phytohemagglutinin or with antiserum against target cell antigens were not cytotoxic for mumps-infected Vero cells. Moreover, the lymphocyte reaction against virus infected target cells could be inhibited by high concentrations of hyperimmune rabbit antimumps sera. On the other hand, lower concentrations of antisera specifically poteniated the lymphoycte-mediated isotope release from mumps-infected target cells.