The intrathymic presence of the muscle acetylcholine receptor (AChR) is controversial, and the nature of the cell(s) expressing it is unclear. We thus analyzed the molecular expression of muscle AChR in human thymi. mRNA studies indicated that the two isoforms (P3A+ and P3A-) of the alpha-subunit were present in thymic extracts and in cultured thymic epithelial cells (TEC), while expression in thymocytes was low and not consistently detectable. The amount of mRNA coding for the alpha-subunit, evaluated by means of quantitative PCR, was about 20 times less in TEC than in muscle, and was similar in TEC from normal subjects and from patients with myasthenia gravis (MG). The beta- and epsilon-subunits present in adult AChR were also expressed in TEC (but not in thymocytes), while the embryonic subunit (gamma) was absent. In TEC cultures, the AChR alpha- and epsilon-subunit mRNA levels were down-regulated by forskolin, as also observed in the TE671 rhabdomyosarcoma cell line, suggesting similar regulation of AChR subunits in thymus and muscle. Protein expression was evidenced on TEC (but not on thymocytes), by Western blotting as well as by immunofluorescence, thus demonstrating AChR expression on human thymic epithelial cells. There was no difference in the expression of AChR between TEC from MG patients and controls, meaning that the expression of AChR subunits alone is not sufficient to explain the onset of MG.