The cytotoxic factor to cultured thyroid cells in sera from patients with autoimmune thyroid diseases was studied by the (86)Rb exchange method which was simple and quantitative. Kinetic analysis of cell destruction indicated that cytolysis was a rapid event and that most lysis occurred during the first 12 hours. The susceptibility of thyroid cells from Graves' thyroids to the cytotoxic factor was variable. Twenty out of 21 serum samples from patients with Hashimoto's disease and 10 out of 12 serum samples from patients with Graves' disease showed cytotoxic effects on thyroid cells in culture. The percentage of killing of sera from patients with Hashimoto's thyroiditis and with Graves' disease was significantly higher than that of normal individuals (p<0.001). The percentage of killing correlated with the anti-microsomal antibody (MCHA) titer (r=0.68, p<0.001), but not with the anti-thyroglobulin antibody (TGHA) titer. Serum absorption studies with various thyroid subcellular fractions revealed that the cytotoxic activity of sera was reduced markedly after absorption with thyroid microsome or plasma membrane fractions, but not with the thyroglobulin fraction, suggesting that the antigen to the cytotoxic factor was expressed in the plasma membrane.