Significant differences in susceptibility to the induction of experimental allergic orchitis (EAO) were found to exist among the various substrains of BALB/c mice. Substrains which were susceptible to disease induction include BALB/cKa, BALB/cByJ, BALB/cWtJ, BALB/cCmcCr, BALB/cCrgl, BALB/cHeA, BALB/cAnNCr, BALB/cPt, BALB/cWehiUnm, BALB/cOrnl and BALB/cArg whereas BALB/cJ and BALB/cSki BoyPt were resistant to EAO. Aspermatogenesis and autoimmune vasitis, two additional lesions characteristic of murine EAO, were found to correlate with susceptibility to autoimmune orchitis. Susceptibility and/or disease resistance did not segregate within any of the evolutionary branches of the BALB/c genealogic tree nor did it correlate with any of the previously described allelic differences distinguishing BALB/c substrains. Therefore, differential disease susceptibility may represent the manifestation of mutational and/or retroviral induced genotypic differences in the regulatory genes controlling testicular autoimmunity.