Immunization of C57BL/6 mice with BALB.B spleen cells in vivo and subsequent boosting in mixed lymphocyte culture result in the generation of cytolytic T lymphocytes (CTLs) which are specific for a limited number of immunodominant antigens. Experiments are described which suggest the existence of a hierarchy of immunodominance in this donor: host combination. Two antigens, CTT-1.3 and CTT-2.3, are dominant in the C57BL/6 anti-BALB.B CTL response. The distribution of these antigens among CXB recombinant inbred (RI) strains suggests that they segregate as single gene traits. Elimination of the CTT-1.3 and CTT-2.3 antigens by complementation in the responder, or elimination from the priming and boosting stages by the selection of CXB RI strain mice as responders or stimulators, reveals a second level of immunodominant antigens which include CTT-3.3 and CTT-4.3. CXB mice which express one of the CTT-1.3 or CTT-2.3 antigens will produce CTLs specific for the other antigen upon priming and boosting with BALB.B cells. Expression of both antigens in responders results in the generation of CTLs specific for the second level, dominant antigens. Immunodominance is not confined to the C57BL/6 anti-BALB.B system but can also be observed in the BALB.B anti-C57BL/6 and B10.D2 anti-DBA/2 systems. Finally, generation of CTLs following priming and boosting with dominant and dominated antigens presented on different cells confirmed that immunodominance can only be observed when the dominant and dominated antigens are presented on the same cells. These observations suggest that immunodominance is revealed at the level of antigen-presenting cells primarily involved in vivo priming.