AbstractThe frequency and specificity of clones of cytotoxic lymphocytes (CL) produced when (CBA × C57BL)F1spleen cell populations were cultured with 2,4,6‐trinitrophenyl (TNP)‐modified syngeneic F1cells, was examined. The frequency of clones which lysed F1‐TNP targets was 1/3.3 × 104spleen cells, and the frequency of clones which lysed the modified parental cells, CBA‐TNP and C57BL‐TNP was 1/6.7 × 104and 1/2.9 × 105spleen cells, respectively. Using a clonal analysis of the specificity of the CL, it was shown that the majority of the clones of CL which lysed the two modified parental targets, were specific for one or the other of the targets. Activity against modified allogeneic DBA/2‐TNP targets was also detected. The activity against DBA/2‐TNP targets was due mainly to clones of CL which were specific for DBA/2‐TNP targets. Only a minor part of the activity was due to clones which were cross‐reactive for both F1‐TNP and