A number of T cell functions were examined in mice after treatment with rabbit anti-mouse thymocyte serum (ATS). Two populations of T cells were distinguished; the first, sensitive, and a second, resistant, to elimination by ATS. The helper cell involved in the primary humoral response to sheep red blood cells (SRBC) was found within the sensitive population, whereas the T-cells required for the priming of mice with SRBC for helper activity or delayed hypersensitivity were found within both populations. Previous work and experiments utilizing both adult thymectomy and ATS treatment indicated that the ATS-sensitive population is long-lived, whereas the ATS-resistant population is short-lived.