Serum or urine from normal adult Lewis rats can be shown to contain detectable amounts of idiotypic, antigen-binding receptors with specificity for the major histocompatibility complex locus antigens of the rat, the Ag-B locus antigens. Such purified naturally occurring receptor molecules, be they of T- or B-lymphocyte origin, can be used in a polymerized form to provoke the production of auto-anti-idiotypic antibodies when injected back into normal Lewis rats. As a consequence of this autoimmunity, lymphocytes of these Lewis rats can be shown to be depleted of cells carrying the relevant idiotypic receptors signifying reactivity against a given Ag-B locus-determined antigen(s). This specific lack of idiotypic lymphocytes is manifested as a selective loss of reactivity against the relevant Ag-B-incompatible antigens as measured by graft versus host or MLC reactions. Furthermore, autoimmune Lewis rats display specific transplantation tolerance against the skin grafts from the relevant strain, as demonstrated by specific prolongation of graft survival. A further indication of the specific tolerence state of these rats comes from the highly reduced ability to produce circulating antibodies against the relevant Ag-B antigens. No side effects of these autoimmunization procedures have been noted so far. It would thus seem clear that a prolonged state of specific transplantation tolerance can be achieved via autoimmunization against the individual's naturally occurring idiotypic, antigen-binding receptors.