The serum inhibitory activities on RBC-C3b receptor rosette of healthy individuals were studied in myasthenia gravis (MG) patients. The serum inhibitory activities appeared in 65% of MG patients. The serum inhibitory activities of generalized type MG were higher than those of ocular type MG. It was also found that the serum inhibitory activities were correlated with the RBC-C3b receptor rosette rate and immune complex (CIC). Furthermore, it was identified that the CIC of MG serums possessed the inhibitory activity on RBC-C3b receptor rosette.