Serum IgM has been shown to participate in the control of IgG autoreactivity in healthy subjects. We have recently shown that an immunoglobulin preparation of pooled normal human IgM (IVIgM) contains anti‐idiotypic antibodies against disease‐associated IgG autoantibodies in autoimmune patients and protects rats from experimental autoimmunity. The aim of the present study was to asses the in vitro and in vivo immunomodulatory effects of IVIgM in comparison with IgG, in SCID mice reconstituted with thymic cells from a myasthenia gravis patient. Non‐leaky SCID mice were injected i.p. with 60 × 10⁶ thymic cells from a patient with myasthenia gravis and 1 day later boosted with 10⁶ irradiated acetylcholine receptor (AchR)‐expressing TE671 cells. On days 14, 21 and 28, mice were treated with IVIgM or with equimolar amounts of human serum albumin. The level of anti‐AchR antibodies in the sera of three out of four IgM‐treated animals was less than 1 nM. Further, there was a significant decrease in the loss of endplate AchR on the diaphragms of IgM‐treated SCID mice. These findings indicate that pooled normal IgM exerts an immunoregulatory role in experimental myasthenia gravis, and suggests that IgM may be considered as an alternative approach in the therapy of autommune diseases.