AbstractA limiting dilution culture system based on stimulation with concanavalin A (Con A) has been used to study the quantitative distribution of helper and of cytotoxic precursor cells in Lyt‐2‐defined subpopulations of murine T cells. Virtually all of the selected Lyt‐2+and Lyt‐2−T cells grow and expand to large clonal colonies within an 8–9‐day culture period. Our data show that upon stimulation with Con A, 90% of the Lyt‐2−T cells were capable to produce interleukin 2 (IL2) activity. In addition, IL2 activity is produced by 8–10% of Lyt‐2+T cells. However, at the clonal level, the average of the IL 2 activity produced by Lyt‐2+T cells is about 8‐fold less as compared to Lyt‐2−T cells. Precursors of cytotoxic T cells were almost exclusively found in the Lyt‐2+population, of which about 70% displayed lytic activity in a lectin‐dependent cytolysis test. For the vast majority of clones analyzed the capacity to produce IL 2 activity and the capacity to express lytic activity, was found to be mutually exclusive. A minority of clones (<3%) was found to simultaneously produce IL2 activity and to express cytotoxicity. These latter cells are therefore c