Previous reports have shown that one cytolytic T lymphocyte (CTL) can kill more than one allogeneic target cell, but have not established whether after its action on one or more target cells, each CTL can attack a second set of target cells at a later time. The experiments described here show that one CTL can kill more than six target cells, and they suggest that CTL can do this sequentially in time. Hence, the small number of specifically sensitized lymphocytes found in rejecting grafts could play an important role. Moreover, sequential killing would imply that CTL are not harmed during their interaction with specific target cells. If killing were effected by secretion of a nonspecific toxin, the killer cell would then have evolved some device for resisting its own toxin in order to survive. Whether or not this is the case has not yet been fully resolved.