We described in this paper systematic alterations in the expression of unique I region controlled epitopes on helper T cells (Th) in chimeras according to the changes in their H-2 restriction specificity. Taking advantage of the reactivity of monoclonal antibodies (anti-Iat) putatively specific for the epitopes indirectly controlled by I region and expressed in association with the Iak restriction site of Th, we examined the alterations of these epitopes on Th cells from various bone marrow chimeras. Iatk epitopes were physiologically expressed on Iak-restricted but not on Iab-restricted Th cells in (H-2k X H-2b)F1 mice. In the chimeric condition, the H-2k-restricted Th of B6----F1 chimera acquired the expression of Iatk even though B6 Th is unable to express Iatk when developed under the physiologic condition. Iatk are also found on Th of fully allogeneic chimera of B6----C3H, whereas Th cells of C3H----B6 completely lost the Iatk expression. These results indicate that Iat epitopes originally defined as unique I region-controlled determinants selectively expressed on T cells are not encoded by the I region genes but are associated with the T cell receptor that sees the self Ia. The epitopes undergo the adaptive alterations according to the acquisition of a new MHC restriction. This is the first example to demonstrate the epitope associated with T cell receptor which undergo the systematic adaptive differentiation.

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