For the production of experimental autoimmune thyroiditis in the mouse comparison of two preparations of complete Freund's adjuvant showed that one, using n-hexadecane and glycerol mono-oleate with freshly harvested tubercle bacilli, was more effective in eliciting both dermal hypersensitivity and thyroid lesions than a mixture of paraffin oil and mannide mono-oleate with dried tubercle bacilli. Both preparations were of comparable efficiency in eliciting thyroid-specific antibodies detected by passive hemagglutination. With either adjuvant, randomly bred CF-1 mice were consistently more susceptible to the induction of thyroid lesions than were inbred C3H mice. When the paraffin oil adjuvant was used, C3H mice developed no lesions despite high titers of circulating antibody.