The memory cytolytic T lymphocyte (CTL) response of mice was studied in vitro using peripheral blood leucocytes (PBL) of donors previously immunized with allogeneic cells as responding cells, and particulate alloantigen for stimulation. It is shown that alloimmune but not normal PBL respond to particulate antigen and that small numbers of PBL generate high CTL responses provided non-T accessory spleen cells are added to the cultures. The system provides means to analyse the kinetics of the appearance of memory CTL precursors after primary alloimmunization in individual mice.