This paper describes a system for determining the frequency and fine specificity of influenza A virus-immune memory cytotoxic T cell (CTL) clones from limiting dilution (LD) microcultures. We found that such experiments can only be performed (1) by analyzing the clonal response of CTL from wells of replica plates containing fractions of one original plate, (2) when it has been ascertained that splitting is possible at the clonal level, and that each fraction of a microculture well gives an identical response on target cells infected with the stimulating virus. With these requirements fulfilled we found that short term CTL clones from LD microcultures from influenza A virus (A/X-31)-immune mice (C57BL/6) occur at a frequency of f = 1:546 to f = 1:6303. The effector cells carry the Lyt-2.2 marker and are specific for target cells infected with the immunizing virus (influenza virus A/X-31). They do not lyse NDV (Newcastle disease virus) or HSV (herpes simplex virus)-infected or NK (YAC) target cells.