The Ly and Ia phenotypes of T lymphocytes involved in the in vitro generation of helper and suppressor cells were identified. The precursors of both cells are found in adult thymectomized spleen. Helper precursors are Ly-1+2-3-Ia-, while suppressor precursors are Ly-1-2+3+Ia-, although the suppressor effector is Ia+. In both cases a second 'amplifier' cell is required for differentiation of precursors to occur. This cell is found in anti-lymphocyte serum-treated spleen and has the phenotype Ly-1+2+3+Ia-.