Cytolytic T Lymphocytes

observations and their interpretation becomes blurred with time. A re-evaluation of the data on CTL may become important at a time when the emphasis of the questions asked about these cells changes. We think the discovery of T-cell growth factor and the design of methods that have allowed the maintenance of CTL clones in permanent culture have ushered in a period in which the established conceptual and experimental framework of cellular immunology is rapidly ceding to a much more cell­ biologically oriented approach to CTL. Therefore, in the second part, we identify some of the questions that arise from the work with CTL clones and that represent this new outlook. Our references are quite eclectic, and for some areas-e.g. lytic mecha­ nisms-we refer mainly to some of the comprehensive reviews that have appeared recently.