The Obese strain (OS) of chickens, which is afflicted with Hashimoto-like spontaneous autoimmune thyroiditis (SAT), displays elevated T cell proliferation, interleukin (IL)2 production and IL2 receptor expression upon mitogen stimulation, and defects in the neuroendocrine control of the immune system including elevated corticosteroid-binding globulin (CBG) and a deficient increase of serum corticosterone (CN) upon cytokine injection. Recently this strain has further been shown to harbor retrovirus-related sequences (endogenous virus no. 22, ev22) absent in healthy control strains. To determine the number of genes responsible for SAT-associated immunodysregulation and to unravel possible ev22 associations, we analyzed the above immune and endocrine parameters in F1 hybrids and backcrosses of the autoimmune OS B15B15 with healthy inbred CB B12B12 chickens. OS-like T cell hyperproliferation and IL2 hypersecretion in response to both concanavalin A and phytohemagglutinin were transmitted as autosomal dominant traits and co-segregated in backcross animals. In vivo hyporesponse of the OS to the corticosterone-inducing effect of cytokine preparations was inherited dominantly and the elevated CBG serum levels recessively. None of these traits appeared to be major histocompatibility complex (MHC) linked. However, while T cell abnormalities and elevated CBG serum levels were not associated with the autosomal ev22 locus, in vivo hyporesponsiveness to glucocortocoid-inducing cytokines co-segregated with this OS-specific provirus. These results add to the concept of SAT as a polyetiological and plurigenetic disease and do not support our previous hypothesis that T cell hyperreactivity and immunoendocrine dysfunction might be functionally related.