The kinetics of the proliferative response and the appearance of effectors of helper activity after stimulation by antigen were examined in T cell subpopulations. As defined in previous papers of this series, one population, T1, is short-lived after adult thymectomy (ATx), and relatively resistant to elimination by anti-thymocyte serum (ATS). Another population, T2, is long-lived after ATx, but highly sensitive to elimination by small doses of ATS. From precursors within the T2 population, effectors of specific helper activity, after priming with antigen, appeared within 1 to 2 days and reached a maximum on day 4. The responding cells reached their peak proliferative response within 24 hr after stimulation by antigen. In contrast, helper activity arising from T1 precursors first appeared on day 3 and peaked on day 5. These cells did not reach their maximal proliferative response until 60 hr after priming. These findings indicate additional useful markers for distinguishing the T1 and T2 subpopulations and are consistent with models for T cell development in which T1 cells are virgin cells and T2 cells are memory cells.