TOLERANCE to self-antigens has been shown to develop during ontogeny as a result of the elonal deletion of self-reactive T cells^1–4. Tolerance, or better 'nonresponsiveness', to specific antigens can also be induced in adult animals^5–9 but the mechanism(s) involved are not well understood¹⁰. Most murine T-helper cells that express the Vβ6 T-cell receptor gene segment are specific for Mls-1^a antigens³. We have therefore been able to use an anti-Vβ6 monoclonal antibody to follow the fate of Mls-1^a specific T cells in adult Mls-1^b mice made specifically unresponsive to Mls-1^a. We show that the induced unresponsiveness is not due to elonal deletion, but rather to elonal anergy. The anergic Vβ6 T-helper cells express IL-2 receptors and undergo limited blastogenesis in vitro upon stimulation, but do not produce IL-2, in marked contrast to Vβ6 cells from naive mice. Our data appear to represent an in vivo correlate for the induction of anergy that has been observed in T-cell lines in vitro^{l l–15}.