We have described a model of immunological tolerance induced, in adult mice, by a single injection of a moderate dose of a hapten-protein conjugate. The data suggest that the mechanism of this tolerance state is the production of small amounts of high affinity antibody in response to the tolerance-inducing antigen injection. This antibody acts to inhibit the response to a subsequent challenge with antigen in complete Freund's adjuvant by a mechanism comparable to that of passive antibody-medicated immune suppression. It was shown that a small but high affinity. Tolerance was not terminated by transfer of normal syngeneic spleen or peritoneal cells into tolerant animals. Spleen cells from tolerant mice, when transferred into lethally irradiated, syngeneic animals, produced a PFC response which is greater in magnitude and tolerance state had a significant degree of carrier specificity which was shown to be comparable to the carrier specificity of antibody-mediated immune suppression. hus, evidence was presented to show that one mechanism of tolerance in adult animals in the suppressive effect of small amounts of high affinity antibody formed in response to the tolerizing injection of antigen.