Sensitized T lymphocytes can specifically destroy in vivo target cells with which they enter into direct contact. This suggests an important role for cytolytic T lymphocytes (CTL) in the destruction of grafts and tumours. Quantitative methods allow the assay of CTL and of CTL memory cells which appear in lymphoid tissues, blood, grafts and tumours during rejection. The formation of CTL and CTL memory cells can also be induced in vitro, and the functional activity of such cells has been demonstrated in transfer experiments. In the present report, results of experiments will be discussed which were aimed at the analysis of the characteristics of memory cells, of the functional activity in vivo of CTL induced in secondary mixed leucocyte cultures (MLC), and of the specificity of CTL formed in response to tumours induced by the Moloney sarcoma virus (MSV). Our results show that there exists a qualitative difference between the original precursor of the CTL and its memory cell, and that the transfer by the intravenous route of CTL induced in secondary MLC protects irradiated mice effectively and specifically against allogeneic tumour growth. In the MSV system, studies of the specificity of CTL show that syngeneic tumour cells are lysed preferentially (and that allogeneic MSV induced tumour cells are not lysed), indicating a genetic restriction in their activity.