Sequential killing of allogeneic target cells by immune cytotoxic T lymphocytes (CTL) was directly observed by time-lapse microcinematography. Target cells (EL4 lymphoma cells from C56BL/6 mice), coated with Fab fragments of goat antibody to EL4, were immobilized by binding to the floor of a polystyrene tissue culture flask that had been precoated with specifically purified anti-goat Fab. On adding immune BALB/c spleen CTL to such target cell monolayers it could be verified by direct observation that individual CTL could sequentially kill several target cells, that the CTL usually separated from the target cell before target cell death, that not all contacted target cells were killed, and that duration of contact was variable and not correlated with subsequent target cell death.