In myasthenia gravis (MG), extraocular muscle (EOM) weakness is often an initial and persisting symptom. It has been proposed that acetylcholine receptor (AChR) from EOM is antigenically different from AChR of other innervated muscles and that the presence of antibodies to fetal AChR expressed in EOM causes their weakness. We have (1) studied mRNA expression for each of the AChR subunits (a, p, y, 6, and E) in human muscle, including EOM, and (2) compared the binding of sera from ocular myasthenia gravis (OMG) patients with fetal (cwJ3ys) and adult ((YJ~ES) human AChRs. RNase protection assays showed that expression of the AChR y-subunit (fetal-type) mRNA in EOM was comparable with that in other innervated muscle types. By contrast, &-subunit (adult-type) mRNA was expressed at much higher levels in EOM than in other muscles studied. Moreover, some OMG sera bound specifically to adult AChR. These results do not support the contention that susceptibility of EOM in MG results from expression of fetal AChR and indicate that the inclusion of antigen from a source rich in adult AChR in the MG diagnostic assay will increase the yield of positive results in OMG patients. bodies to both en grappe and en plaque end plates of rat EOM. O n the other hand, other workers have found that M G sera, which react preferentially with AChR derived from normal innervated human leg From the