Immunoprecipitation of human acetylcholine receptor (AChR) is used in the diagnostic assay for myasthenia gravis (MG). We compared human AChR derived from TE671 cells, which express fetal-type AChR, with AChR from TE671-epsilon cells, which we have engineered to express adult-type AChR. Some low-titer MG sera distinguished strikingly between the two subtypes. Four out of seven MG sera that had equivocal titers in standard assays gave positive titers with TE671-epsilon AChR, whereas only one out of seven gave a positive titer with TE671 cells. The new cell line provides a greater concentration of adult AChR than can be obtained from normal human muscle and increases the sensitivity of the diagnostic assay.