Anti-acetylcholine receptor (AChR) antibody (ab) was found in the serum of a dog with acute myasthenia gravis (MG) by the use of Cowan 1 strain Staphylococcus aureus to bind radiolabeled anti-AChR ab-AChR immune complexes. Fifteen months later, when the dog was in remission, there was only a very low level of the antiAChR ab. These observations strengthen the contention that anti-AChR ab is important in the pathophysiology of myasthenia gravis. Higher titers of the canine ab were measured with rabbit than with human AChR, whereas 17 human MG sera, selected to represent a wide range of anti-AChR ab titers, were all more reactive with human AChR. The degree of cross-reactivity of human anti-AChR ab with rabbit AChR varied widely, indicating a heterogeneous population of anti-AChR ab molecules in human myasthenia gravis sera.