Inbred strains of mice were studied for their susceptibility to the induction of experimental allergic orchitis after sensitization with mouse testicular homogenate in complete Freund's adjuvant accompanied by injections of extract from Bordetella pertussis. Susceptibility to autoimmune orchitis was found to be linked to the major histocompatibility complex in BALB/c and C57BL/10 mice and mapped to genes encoded within the H-2Dd region. In five of six groups of bidirectional (susceptible X resistant)F1 hybrids, H-2Dd-linked susceptibility was inherited as a dominant autosomal trait. However, in (BALB/cByJ X DBA/2J)F1 and (DBA/2J X BALB/cByJ)F1 hybrids, dominant autosomal resistance to the induction of autoimmune orchitis was observed. Backcross analysis between the resistant F1 hybrid and the susceptible BALB/cByJ parent suggests that a single independently segregating DBA/2J locus is capable of negating H-2Dd-linked susceptibility, and controls resistance to the induction of autoimmune orchitis.