Mice were given intraperitoneal injections of serum from three patients with myasthenia gravis who had different titers of antibody against mouse acetylcholine receptor (AChR). Almost all mice treated with a high titer of serum antigens showed generalized paralysis, reduced amplitudes of miniature endplate potential (MEPP), or reduced numbers of AChRs. The effects were less marked in serum with lower titers of antibody to mouse AChR, when there was no relation to the severity of effects to the titer of antibody to human AChR.