Summary By the methods which have been successfully used in a study of the etiology of other diseases a streptococcus of low general virulence, having characteristic cataphoretic velocity, has been found constantly associated with cases of myasthenia gravis. The living and heat-killed streptococcus and its toxins have specific affinity for muscles. The progressive weakness, fatigability, and the pathologic lesions of myasthenia gravis have been reproduced in monkeys and rabbits.