Acetylcholine receptors are present in the sarcolemma of cultured skeletal muscle myotubes either as large clusters or in a diffuse distribution. Both the clustered and diffuse acetylcholine receptors are potentially removable from the membrane. Treatment of myotubes with globulin from patients with myasthenia gravis causes the loss of acetylcholine receptor clusters and the concomitant appearance of acetylcholine receptor microaggregates. The rate of acetylcholine receptor cluster loss is greater than the rate of acetylcholine receptor degradation, indicating that acetylcholine receptors are disrupted from clusters to form microaggregates before being removed from the plasma membrane.