In an attempt to define further the activation of effector T-cell function with concanavalin A (Con A), we examined the ability of Con A to generate effector memory in mouse lymph node cells in vitro. In the course of these studies, it became necessary to define parameters by which memory could be defined. These parameters include length of time of exposure to signal required to generate full cytotoxic function; time of onset and kinetics of blast cell formation; requirement for DNA synthesis; sensitivity to the drug 5'-bromodeoxyuridine; and kinetics of the cytotoxic response to alloantigen. By these criteria, mouse lymph node cells exposed 12 days previously to Con A behave qualitatively differently from unprimed mouse lymphocytes. We found that the time of onset and kinetics of DNA synthesis could not be used to distinguish primary and secondary cytotoxic responses. We propose that the parameters defined in these stidues can be applied generally in determining whether a given cytotoxic response involves primed (memory) or unprimed cells.