Investigations of cellular cytotoxicity of the immune system are hampered by the lack of continuously growing, transformed cell lines which express a cytotoxic potential. Here we describe cytotoxic cell lines from the cotton-topped marmoset monkey, transformed by Herpesvirus Ateles (HVA) or Herpesvirus Saimiri (HVS), which can kill certain target cells in a short-term in vitro test. HVA/HVS-transformed cells have earlier been classified as belonging to the T-cell lineage in contrast to Epstein-Barr virus (EBV)-transformed cells derived from B lymphocytes. We suggest that the HVA/HVS-transformed killer cell lines described here represent an effector population resembling, or corresponding to, marmoset natural killer (NK) cells and that they may be used to define the cytolytic mechanism involved in cellular cytotoxicity and possibly also effector cell receptors and target cell antigens, as well as regulatory mechanisms of general biological interest.