Guinea pigs were immunized once with homologous thyroglobulin in complete Freund’s adjuvant and observed for lesions in the thyroid gland as well as for different parameters of the immune response to thyroglobulin. In individual animals there was a significant correlation between the development of thyroiditis and delayed type skin hypersensitivity to thyroglobulin and the inhibition of migration of peritoneal exudate cells by thyroglobulin. Thyroid lesions and haemagglutinating antibodies to thyroglobulin were less well correlated. Lymphoid spleen cells from immunized animals lysed thyroglobulin coated chicken erythrocytes. Sera from these animals contained antibodies which in the absence of complement rendered lymphocytes from normal guinea pigs cytotoxic for thyroglobulin coated chicken erythrocytes. Presence of thyroid lesions was correlated to both these phenomena. The incidence of thyroiditis and cytotoxicity of immune spleen cells reached their peak 3–4 weeks after immunization and then decreased. The inhibition of macrophage migration reached its peak before the establishment of thyroid lesions and decreased at the same time as the incidence of thyroiditis decreased. Skin reactivity, haemagglutinating antibodies and antibody induced cytotoxicity of normal lymphocytes remained unchanged during the observation period.