Chicken erythrocytes were labelled with 51Cr and treated with Fab-fragment of rabbit anti-ChRBC IgG antibodies conjugated with human thyroglobulin. When incubated with sera from patients with Hashimoto thyroiditis and highly purified peripheral blood lymphocytes from normal donors or guinea pigs, the red cells were lysed, as assessed by isotope release. The time course of the reaction was slow but reached high levels at 72 h of incubation. The activity of the sera was not correlated to their titre of haemagglutinating antibodies to thyroglobulin.