To improve the sensitivity of the radioimmunoassay method for anti-AChR-antibody, large amounts of sera from patients with myasthenia gravis, and higher concentrations of antigens and rabbit anti-human-IgG-antiserum, were used. These procedures enabled measurement of the titre value of over 0.04 pmol/ml serum and this value revealed a sensitivity about 10 times higher than that predicted using the previous model. Antibodies against AChR were found in 13 out of 17 ocular myasthenia patients (70%) and 35 of 37 with generalized myasthenia (90%). However, the average titre value of sera in those with ocular myasthenia was significantly lower than the value obtained in the generalized cases. Even in the patients with ptosis or ophthalmoparesis, in the early stage (less than one year) of ocular myasthenia, anti-AChR-antibodies were not detectable using this more sensitive assay method.