TUMOURS of antibody-secreting B lymphocytes (myeloma tumours) have contributed greatly to our understanding of antibody structure and function. T-lymphocyte tumours that retain effector activities might, if they exist, prove to be similarly useful for analysing T cells and their key components. The recent finding that a T lymphoma is capable of crawling on other cells, like antigen-activated normal T lymphocytes1, prompted us to investigate whether some lymphocytic tumour cells can, like normal cytotoxic T lymphocytes, lyse target cells. We report here that they can.