SummaryThe natural killer (NK)*cell induced one to two days after Kunjin virus infection of BAI.B/c mice is cytotoxic for a wide range of syngeneic, allogeneic and xenogeneic cell lines. It is also weakly cytotoxic for some non‐malignant cells including mouse fibroblasts, macrophages and thymocytes, but not lymph node cells. Levels of lysis of non‐tumour target cells are dependent on their genotype. Furthermore, malignant cell lines may become resistant following transplantationin vivothen susceptible again after culturein vitro.The virus‐induced NK cell is elicited as readily in athymic (nude) as in normal mice. X‐irradiation inhibits its development if administered prior to infection. It is labile on culture at 37°. The cell carries Fc receptors but its NK activity is not antibody‐dependent.