An antiserum specific for cytotoxic T lymphocytes (CTL) and their precursors has been prepared by immunization of strain 13 guinea pigs with a partially purified population of splenic CTL from BALB/c mice immunized with the C57 T cell lymphoma EL4. After absorption with EL4 cells, this antiserum (in the presence of C) totally abolished the activity of in vivo generated CTL and CTL precursors, although it killed only 20% of total Ig- spleen cells. Antibody activity was independent of the H-2 or Ly allotypes of the cells being treated. Helper activity and proliferative responses to concanavalin A and to phytohemagglutinin were unaffected. There was a variable effect on the mixed lymphocyte reaction, and a partial inhibition of graft-vs-host activity.