AbstractMurine cytotoxic T lymphocytes (CTL) were induced in “one‐way” mixed lymphocyte cultures and their physical characteristics investigated by velocity sedimentation at 1 × g. Thein vitrodifferentiation of progenitors of CTL into primary and secondary CTL was paralleled by characteristic changes in the size of the responder cells. Fractionated cells enriched for primary blast CTL reverted into clonally restricted “nonlytic” secondary T lymphocytes. Upon antigenic reexposure, these lymphocytes differentiated into secondary CTL within 18–32 h. This took place in the absence of cell proliferation and could be triggered by UV light irradiated allogeneic stimulator cells. It is suggested that the different characteristics for the induction of either a primary or secondary cytotoxic T cell response reflect qualitative differences between unprimed T cells and memory T