To examine the role played by adhesion molecules in thyroid autoimmunity, we have assessed the effect of administering monoclonal antibodies (mAb) against intercellular adhesion molecule-1 (ICAM-1) and lymphocyte function-associated antigen-1 (LFA-1) in experimental autoimmune thyroiditis, induced by immunizing rats with thyroglobulin in complete Freund's adjuvant. The antibody against LFA-1, but not against ICAM-1, reduced thyroglobulin antibody production (P < 0.01) and both antibodies caused a significant reduction (P < 0.002) in the severity of the thyroidal lymphocytic infiltration. In vitro, both mAb impaired the proliferative response of splenic and lymph node T cells to thyroglobulin, but only the antibody against LFA-1 reduced thyroid cell killing assessed using splenic lymphocytes as effectors. Monoclonal antibodies against both these adhesion molecules appear to inhibit cell-mediated autoimmunity in vivo, but only the LFA-1 mAb reduced the autoantibody response.