ANIMALS immunised with acetylcholine receptor (AChR) purified from the electric organ of Electrophorus electricus or Torpedo marmorata develop a muscular weakness which is alleviated by injections of anticholinesterases^1–6. This observation has provided a valuable animal model for study of the disease myasthenia gravis. Sera from rats immunised with electric eel receptor contain antibody to AChR isolated from rat skeletal muscle^2,6, suggesting that the muscular weakness is due to a reduced endplate sensitivity to ACh. We report here a reduction in the ACh sensitivity of denervated muscles from immunised rats. Furthermore, there is a factor in the serum of immunised rats which can reduce the ACh sensitivity of denervated rat muscles in vitro.