In 82 white patients with myasthenia gravis, a high serum human acetylcholine receptor (AChR) antibody titer was related to the presence of the HLA-B8 antigen and increasing severity of the disease and not to age at onset, sex, presence of thymoma, or mode of treatment. Among patients without thymomas a high antibody titer was also associated with HLA-B8, particularly in those patients whose age at onset was less than 35 years. Thymectomy was associated with a lower median antibody titer when compared in two groups of HLA-B8-positive patients without thymoma who were similar for all other factors. Patients with thymomas who had received corticosteroids had a lower median titer than those who had not received steroids. This study supports the possibility that immune-response genes near the HLA-B8 segment of the major histocompatibility complex participate in the regulation of the humoral response to autoantigens, such as AChR protein.