The main immunogenic region (MIR) of the acetylcholine receptor (AChR) is the target for the majority of high-affinity autoantibodies produced in myasthenia gravis patients. Some monoclonal antibodies (mAbs) to the MIR bind specifically, but with low affinity, to synthetic AChR alpha subunit peptides with the sequence alpha 67-76. Studies of synthetic peptides suggest that amino acids alpha 68 and alpha 71 may be especially important to the antigenic structure of the MIR. We have studied the contribution of amino acids alpha 68 and alpha 71 to the antigenicity of the MIR on intact AChR by replacing alpha 68 (N) and alpha 71 (D) of Torpedo AChR alpha with alpha 68 (D) and alpha 71 (K) by site-directed mutagenesis, expressing the mutated transcripts in Xenopus oocytes along with wild-type Torpedo beta, gamma and delta subunits, and analyzing the expressed AChR for the binding of mAbs to the MIR. These mutations of the MIR greatly diminished binding of mAbs to the MIR. Thus, both alpha 68 and alpha 71 are critical to the antigenicity of the MIR in intact AChRs.