Previous studies from our laboratory showed that B 10.A mice are high responders to pigeon cytochrome c fragment 81-104, whereas'B 10.A(5R) mice are low responders. In the present studies, the C-terminal cyanogen bromide cleavage fragment and homologous synthetic peptides of tobacco horn worm moth cytochrome c were shown to be immunogenic in both B10.A and B10.A(5R) mice. These strains, however, showed different patterns of cross-reactivity when immune lymph node T cells were stimulated with cytochrome c fragments from other species. To examine the two patterns of responsiveness at a clonal level, cytochrome c fragment-specific T cell hybridomas were made and found to secrete interleukin 2 in response to antigen. The patterns of cross- reactivity of these B 10.A and B 10.A(5R) clones were similar to that seen in the whole lymph node population. Surprisingly, when these clones were tested for major histocompatibility complex (MHC)-restricted antigen recognition, they were all found to respond to antigen with both B10.A and B10.A(5R) antigen-presenting cells (APC). Furthermore, the cross-reactivity pattern appeared to be largely determined by the genotype of the APC, not the genotype of the T cell clone. That is, a given T cell clone displayed a different fine specificity when assayed with B10.A or B10.A(5R) APC. This observation indicates that the APC MHC gene product and antigen interact during the stimulation of the T cell response and that as a consequence the specificity of antigen-induced T cell activation is influenced by these MHC gene products. (During the preparation of this manuscript it has come to our attention that results similar to our own, concerning the fine specificity of cytotoxic T cell clones, have been obtained by Dr. T. R. Hunig and Dr. M. J. Bevan, Massachusetts Institute of Technology, Boston, MA. T. R. Hunig and M. J. Bevan. 1981. Specificity of T-cell clones illustrates altered self hypothesis. Nature. 294:460.)