Muscle weakness in myasthenia gravis is due to autoantibody‐induced loss of functional acetylcholine receptors (AChR). About 15% of myasthenia gravis patients, however, do not have detectable anti‐AChR antibodies. To investigate the effect of their plasma immunoglobulins on neuromuscular transmission, mice were injected with plasma (and in some cases purified immunoglobulin G (IgG)) from 7 “seronegative” myasthenia gravis (SMG) patients, and neuromuscular transmission parameters were examined. When injected for 15 days, all patients' plasma caused reductions in miniature endplate potential amplitudes, while endplate potential quantal content was significantly reduced by plasma from 4 of the 7 patients. There were no changes in ACh‐induced depolarization or single channel properties, and ₁₂₅l‐α‐bungarotoxin binding studies showed no effect on AChR number, except in 1 case. Purified IgG injected for 3 days had similar effects to plasma injected for 15 days. Our findings confirm that SMG is autoantibody mediated and that there are pathogenic IgG antibodies. SMG appears to be a heterogeneous disorder and the target(s) for the antibodies may be diverse. © 1994 John Wiley & Sons, Inc.