A dot immunobinding assay (DIBA) for thyrotropin (TSH) receptor antibodies is described. The method depends on the detection of antibody binding to highly purified thyroid plasma membrane attached to nitrocellulose solid support by horse-radish peroxidase - conjugated anti-human IgG. The method can detect down to 0.75 mU LATS standard and 1/1000 dilution of Graves' serum or immunoglobulin fraction. The interaction is inhibited dose-dependently by bTSH but not by insulin or human chorionic gonadotropin. The DIBA results show close correlation to those of TRAK (TBII) but not to cyclic AMP generation assay. DIBA is reproducible when tested monthly for 4 months. Sera and immunoglobulins gave virtually the same results. The method has a sensitivity of 90%, validity of 90% and specificity of 80% for both. We have, thus developed a sensitive and reliable method for screening for TSH receptor antibodies which can be performed in routine clinical laboratories.