Different forms of cellular cytotoxicity that may constitute potential effector mechanisms in the spontaneous autoimmune thyroiditis (SAT) arising in Obese strain (OS) chickens have been investigated. A microcytotoxicity assay (MCA) employing 51Cr-labelled-, thyroglobulin (Tg) coated-chicken red blood cells (Tg-CRBC) was used to detect cells mediating Tg-specific direct cellular cytotoxicity (DCC). Tg-CRBC presensitized with anti-Tg autoantibody (Tg-AAB) obtained from high titer OS sera served as targets for antibody-dependent cell-mediated cytotoxicity (ADCC). Tannic acid-only treated CRBC (TA-CRBC) were used simultaneously as specificity controls for DCC and also as targets for cells eliciting spontaneous cellular cytotoxicity (SCC) to surface-modified normal cells. The results demonstrated that Tg-specific cytotoxic cells exist in the OS and thus represent an effector mechanism in SAT, in addition to the previously well-documented role of antibody. This DCC appears to be unrelated to the presence and titer of circulating Tg-AAb. It is present in highest levels in the peripheral blood of OS chickens, but in only 55% of the animals tested, indicating either a secondary importance to humoral immunity in the disease process or the possibility of different effector mechanisms prevailing in different birds. There were no overall differences in ADCC between OS and normal chickens when the two strains were considered as a whole. Chronological analysis, however, revealed very low ADCC in the peripheral blood of young OS birds, followed by a later elevation above that in the normal control chickens. Destruction of the gland by ADCC may be initiated via maternally-derived of in situ-produced anti-Tg antibody. No overall significant differences in SCC were observed between OS and normal chickens.