The human cell line TE671 produces large amounts of muscle nicotinic acetylcholine receptor (AChR). TE671 cells were used to determine the specificity of antibodies which can increase the internalization rate of AChR (antigenic modulation) and to test procedures for protecting AChR against this mechanism. The half-life of AChR both in the absence and the presence of anti-AChR antibodies was very similar to that of AChR on human muscle cell cultures. The relative contribution of different anti-AChR antibody fractions to the total antigenic modulation capacity of human myasthenic sera was investigated by competition experiments between Fab fragments of anti-AChR monoclonal antibodies (MoAbs) and intact antibodies (MoAb or myasthenic sera). Fab fragments, which do not induce antigenic modulation, were allowed to shield the corresponding regions of the AChR. Intact antibodies were subsequently added. It was found that protection of the main immunogenic region (MIR), but not of a region on the beta-subunit, essentially blocked the modulatory effect of the intact anti-MIR MoAbs, and approximately 80% of that of myasthenic sera. These data suggest that anti-MIR antibodies are mainly responsible for the loss of human AChR via antigenic modulation. Furthermore the observation that Fab fragments of anti-MIR MoAbs can efficiently protect AChR against antigenic modulation may have therapeutic implications.