The effect of 4 to 5 intraperitoneal and subcutaneous injections of guinea pig thyroid extract in saline on the development of experimental allergic thyroiditis in the guinea pig was studied; thyroiditis was induced by the injection of homologous thyroid extract in complete Freund's adjuvant. When these “nonsensitizing” injections were given either before, during or following the challenge injection, there was a marked, but temporary, suppression of thyroiditis at 12 days following sensitization, followed by a rapid recovery to control levels during the following 2 weeks. There was also a corresponding temporary suppression of delayed skin reactions and antibody to homologous thyroglobulin, which, in the group treated prior to sensitization, was also temporary. No suppression was obtained with comparable injections of bovine thyroid extract in saline. The relationship between this form of suppression and other forms of immunologic unresponsiveness in discussed.