To examine the role of T-cell subsets in the development of thyroid lesions, female CBA/J mice were immunized with 60 micrograms mouse thyroglobulin (MTg) in 0.1 ml complete Freund's adjuvant in both hind footpads. The thyroids were removed 12-21 days later, pooled, and dispersed. The cell suspension was examined by membrane immunofluorescence for the distribution of Thy-1+, Lyt-1+, Lyt-2+, and sIg+ lymphocytes. For comparison, peripheral blood leukocytes (PBL) from the same animals were similarly examined. Throughout this 10-day interval, B cells in the thyroid were consistently below 5%, whereas B cells represented 19-24% of PBL. Thy-1+ cells in PBL ranged from 45 to 59%, whereas Thy-1+ cells in the thyroid were 37-50%. However, only thyroidal T cells showed a consistent decline with time and were replaced gradually by cells without T or B cell markers. In particular, there was a clear shift in the Lyt-1+:Lyt-2+ ratio from about 7 down to 2 in the thyroid as the early predominance of Lyt-1+ cells was followed by a relative increase in Lyt-2+ cells. Our results show that there is an accumulation of Lyt-1+ and Lyt-2+ cells in the infiltrated thyroid. These cells may include MTg-reactive, helper, and cytotoxic T cells which localize (or differentiate) in the thyroid and initiate the lesions.