The specificity of recognition of H-2 antigens by various subsets of Tc cells was investigated with respect to the two separate molecules known to be coded in the H-2D(d) region (a) D which carries the private specificity H-2.4 and (b) D' which carries the public specificity H-2.28. BALB/c.H-2(db) mutant mice express D but not D' on their cell surfaces, whereas wild-type BALB/c mice express both D and D'. H-2 restricted Tc cells specific for viral-plus- H-2D(d)-coded antigens on infected self cells, or minor H-plus-H-2D(d)-coded antigens on H-2-compatible cells apparently recognize D, but do not detectably recognize D. In contrast, BALB/c-H-2(db) anti-BALB/c Tc cell responses do recognize D' (the only known antigen which is not shared by mutant and wild-type); furthermore, D' is also detectably recognized by a significant proportion of the Tc cells that respond in MLR to H-2D(d)-coded antigens. In these latter responses, D' was recognized separately from D, i.e., the response was not "H-2 restricted". These results indicate that H-2 restricted Tc cell responses to modified-self cells are more specific for self H-2D(d)-coded antigens then are allogeneic Tc cell responses directed at the same antigens, in that haplotype-unique (private) specificity recognition (of the D molecule) exclusively occurs only in the former, not the latter case. The implications of this specificity of H-2 restricted responses for possible processes of somatic selection of anti-self recognition structures on progenitor Tc cells are briefly discussed.

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