Splenic lymphocytes from four C57BL/10 congenic resistant mouse strains were sensitized in vitro with trinitrophenyl (TNP)-modified autologous spleen cellsmthe effector cells generated were incubated with 51-Cr-labeled unmodified or TNP-modified spleen or tumor target cells, and the percentage of specific lympholysis determined. The results obtained using syngeneic-, congenic-, recombinante, and allogeneic-modified target cells indicated that TNP modification of the target cells was a necessary but insufficient requirement for lympholysis. Intra-H-2 homology either between modified stimulating cells and modified target cells or between responding lymphocytes and modified target cells was also important in the specificity for lysis. Homology at the K serological region or at K plus I-A in the B10.A and B10BR strains, and at either the D serological region or at some other region (possibly K) in the B10.D2 and C57BL/10 strains were shown to be necessary in order to detect lympholysis. Experiments using (B10itimes C57BL/10)F1 responding lymphocytes sensitized and assayed with TNP-modified parental cells indicated that the homology required for lympholysis was between modified stimulating and modified target cellsmthe possibility is raised that histocompatibility antigens may serve in the autologous system as cell surface components which are modified by viruses or autoimmune complexes to form cell-bound modified-self antigens, which are particularly suited for cell-mediated immune reactions. Evidence is presented suggesting that H-2-linked Ir genes are expressed in the TNP-modified autologous cytotoxic system. These findings imply that the major histocompatibility complex can be functionally involved both in the response potential to and in the formation of new antigenic determinants involving modified-self components.