Strain 13 histocompatible guinea pigs developed allergic thyroiditis after immunization with thyroid extracts derived from the same strain or from other strains of guinea pigs. This thyroiditis appeared as early as 5 days after immunization, and by 7 weeks was uniformly present and generally severe. 7 weeks after immunization, the anti-thyroid antibody titer correlated with the presence and degree of thyroiditis. However, at certain other times after immunization, the titer did not correlate with the thyroiditis. By contrast, all animals with thyroiditis, which were skin-tested with thyroid extract, exhibited delayed hypersensitivity. Moreover, all those which failed to respond with delayed reactions, when skin-tested, had not developed thyroiditis. The present work correlates the presence of experimental allergic thyroiditis with delayed hypersensitivity.